There are two issues here:

1. the task was locked, and didn't allow future updates to RSS to work? IMHO RSS task should never lock?
2. the actual error in the trace

 Error performing CreateRssNotifications (Job ID: 77f4bc3d-3a6a-4f9d-8959-5b0745e5f513) from Dynflow(default) in 2249.06ms: ActiveJob::SerializationError (Can't serialize a Hash with reserved key "_aj_symbol_keys"):
 | /home/foreman/gems/gems/activejob-5.2.1/lib/active_job/arguments.rb:131:in `serialize_hash_key'
 | /home/foreman/gems/gems/activejob-5.2.1/lib/active_job/arguments.rb:106:in `block in serialize_hash'
 | /home/foreman/gems/gems/activejob-5.2.1/lib/active_job/arguments.rb:105:in `each'
 | /home/foreman/gems/gems/activejob-5.2.1/lib/active_job/arguments.rb:105:in `each_with_object'
 | /home/foreman/gems/gems/activejob-5.2.1/lib/active_job/arguments.rb:105:in `serialize_hash'
 | /home/foreman/gems/gems/activejob-5.2.1/lib/active_job/arguments.rb:64:in `serialize_argument'
 | /home/foreman/gems/gems/activejob-5.2.1/lib/active_job/arguments.rb:34:in `block in serialize'
 | /home/foreman/gems/gems/activejob-5.2.1/lib/active_job/arguments.rb:34:in `map'
 | /home/foreman/gems/gems/activejob-5.2.1/lib/active_job/arguments.rb:34:in `serialize'
 | /home/foreman/gems/gems/activejob-5.2.1/lib/active_job/core.rb:147:in `serialize_arguments'
 | /home/foreman/gems/gems/activejob-5.2.1/lib/active_job/core.rb:135:in `serialize_arguments_if_needed'
 | /home/foreman/gems/gems/activejob-5.2.1/lib/active_job/core.rb:88:in `serialize'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/active_job/queue_adapter.rb:12:in `enqueue_at'
 | /home/foreman/gems/gems/activejob-5.2.1/lib/active_job/enqueuing.rb:51:in `block in enqueue'
 | /home/foreman/gems/gems/activesupport-5.2.1/lib/active_support/callbacks.rb:109:in `block in run_callbacks'
 | /home/foreman/gems/gems/activejob-5.2.1/lib/active_job/logging.rb:17:in `block (3 levels) in <module:Logging>'
 | /home/foreman/gems/gems/activejob-5.2.1/lib/active_job/logging.rb:48:in `tag_logger'
 | /home/foreman/gems/gems/activejob-5.2.1/lib/active_job/logging.rb:16:in `block (2 levels) in <module:Logging>'
 | /home/foreman/gems/gems/activesupport-5.2.1/lib/active_support/callbacks.rb:118:in `instance_exec'
 | /home/foreman/gems/gems/activesupport-5.2.1/lib/active_support/callbacks.rb:118:in `block in run_callbacks'
 | /home/foreman/gems/gems/activesupport-5.2.1/lib/active_support/callbacks.rb:136:in `run_callbacks'
 | /home/foreman/gems/gems/activejob-5.2.1/lib/active_job/enqueuing.rb:49:in `enqueue'
 | /home/foreman/gems/gems/activejob-5.2.1/lib/active_job/configured_job.rb:15:in `perform_later'
 | /home/foreman/git/foreman/app/jobs/create_rss_notifications.rb:6:in `perform'
 | /home/foreman/gems/gems/activejob-5.2.1/lib/active_job/execution.rb:39:in `block in perform_now'
 | /home/foreman/gems/gems/activesupport-5.2.1/lib/active_support/callbacks.rb:109:in `block in run_callbacks'
 | /home/foreman/gems/gems/i18n-1.1.0/lib/i18n.rb:284:in `with_locale'
 | /home/foreman/gems/gems/activejob-5.2.1/lib/active_job/translation.rb:9:in `block (2 levels) in <module:Translation>'
 | /home/foreman/gems/gems/activesupport-5.2.1/lib/active_support/callbacks.rb:118:in `instance_exec'
 | /home/foreman/gems/gems/activesupport-5.2.1/lib/active_support/callbacks.rb:118:in `block in run_callbacks'
 | /home/foreman/gems/gems/activejob-5.2.1/lib/active_job/logging.rb:26:in `block (4 levels) in <module:Logging>'
 | /home/foreman/gems/gems/activesupport-5.2.1/lib/active_support/notifications.rb:168:in `block in instrument'
 | /home/foreman/gems/gems/activesupport-5.2.1/lib/active_support/notifications/instrumenter.rb:23:in `instrument'
 | /home/foreman/gems/gems/activesupport-5.2.1/lib/active_support/notifications.rb:168:in `instrument'
 | /home/foreman/gems/gems/activejob-5.2.1/lib/active_job/logging.rb:25:in `block (3 levels) in <module:Logging>'
 | /home/foreman/gems/gems/activejob-5.2.1/lib/active_job/logging.rb:48:in `tag_logger'
 | /home/foreman/gems/gems/activejob-5.2.1/lib/active_job/logging.rb:22:in `block (2 levels) in <module:Logging>'
 | /home/foreman/gems/gems/activesupport-5.2.1/lib/active_support/callbacks.rb:118:in `instance_exec'
 | /home/foreman/gems/gems/activesupport-5.2.1/lib/active_support/callbacks.rb:118:in `block in run_callbacks'
 | /home/foreman/gems/gems/activesupport-5.2.1/lib/active_support/callbacks.rb:136:in `run_callbacks'
 | /home/foreman/gems/gems/activejob-5.2.1/lib/active_job/execution.rb:35:in `perform_now'
 | /home/foreman/gems/gems/activejob-5.2.1/lib/active_job/execution.rb:18:in `perform_now'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/active_job/queue_adapter.rb:42:in `run'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/action.rb:538:in `block (3 levels) in execute_run'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/middleware/stack.rb:26:in `pass'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/middleware.rb:18:in `pass'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/action/progress.rb:30:in `with_progress_calculation'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/action/progress.rb:16:in `run'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/middleware/stack.rb:22:in `call'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/middleware/stack.rb:26:in `pass'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/middleware.rb:18:in `pass'
 | /home/foreman/gems/bundler/gems/foreman-tasks-0c53d11a5b2b/app/lib/actions/middleware/keep_current_user.rb:15:in `block in run'
 | /home/foreman/gems/bundler/gems/foreman-tasks-0c53d11a5b2b/app/lib/actions/middleware/keep_current_user.rb:43:in `restore_curent_user'
 | /home/foreman/gems/bundler/gems/foreman-tasks-0c53d11a5b2b/app/lib/actions/middleware/keep_current_user.rb:15:in `run'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/middleware/stack.rb:22:in `call'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/middleware/stack.rb:26:in `pass'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/middleware.rb:18:in `pass'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/middleware.rb:31:in `run'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/middleware/stack.rb:22:in `call'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/middleware/world.rb:30:in `execute'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/action.rb:537:in `block (2 levels) in execute_run'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/action.rb:536:in `catch'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/action.rb:536:in `block in execute_run'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/action.rb:451:in `block in with_error_handling'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/action.rb:451:in `catch'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/action.rb:451:in `with_error_handling'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/action.rb:531:in `execute_run'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/action.rb:278:in `execute'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/execution_plan/steps/abstract_flow_step.rb:17:in `block (2 levels) in execute'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/execution_plan/steps/abstract.rb:162:in `with_meta_calculation'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/execution_plan/steps/abstract_flow_step.rb:16:in `block in execute'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/execution_plan/steps/abstract_flow_step.rb:30:in `open_action'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/execution_plan/steps/abstract_flow_step.rb:15:in `execute'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/director.rb:43:in `execute'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/executors/parallel/worker.rb:12:in `block in on_message'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/executors.rb:12:in `run_user_code'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/executors/parallel/worker.rb:11:in `on_message'
 | /home/foreman/gems/gems/concurrent-ruby-edge-0.2.4/lib/concurrent/actor/context.rb:46:in `on_envelope'
 | /home/foreman/gems/gems/concurrent-ruby-edge-0.2.4/lib/concurrent/actor/behaviour/executes_context.rb:7:in `on_envelope'
 | /home/foreman/gems/gems/concurrent-ruby-edge-0.2.4/lib/concurrent/actor/behaviour/abstract.rb:25:in `pass'
 | /home/foreman/gems/gems/dynflow-1.1.0/lib/dynflow/actor.rb:26:in `on_envelope'
 | /home/foreman/gems/gems/concurrent-ruby-edge-0.2.4/lib/concurrent/actor/behaviour/abstract.rb:25:in `pass'
 | /home/foreman/gems/gems/concurrent-ruby-edge-0.2.4/lib/concurrent/actor/behaviour/awaits.rb:15:in `on_envelope'
 | /home/foreman/gems/gems/concurrent-ruby-edge-0.2.4/lib/concurrent/actor/behaviour/abstract.rb:25:in `pass'
 | /home/foreman/gems/gems/concurrent-ruby-edge-0.2.4/lib/concurrent/actor/behaviour/sets_results.rb:14:in `on_envelope'
 | /home/foreman/gems/gems/concurrent-ruby-edge-0.2.4/lib/concurrent/actor/behaviour/abstract.rb:25:in `pass'
 | /home/foreman/gems/gems/concurrent-ruby-edge-0.2.4/lib/concurrent/actor/behaviour/buffer.rb:38:in `process_envelope'
 | /home/foreman/gems/gems/concurrent-ruby-edge-0.2.4/lib/concurrent/actor/behaviour/buffer.rb:31:in `process_envelopes?'
 | /home/foreman/gems/gems/concurrent-ruby-edge-0.2.4/lib/concurrent/actor/behaviour/buffer.rb:20:in `on_envelope'
 | /home/foreman/gems/gems/concurrent-ruby-edge-0.2.4/lib/concurrent/actor/behaviour/abstract.rb:25:in `pass'
 | /home/foreman/gems/gems/concurrent-ruby-edge-0.2.4/lib/concurrent/actor/behaviour/termination.rb:55:in `on_envelope'
 | /home/foreman/gems/gems/concurrent-ruby-edge-0.2.4/lib/concurrent/actor/behaviour/abstract.rb:25:in `pass'
 | /home/foreman/gems/gems/concurrent-ruby-edge-0.2.4/lib/concurrent/actor/behaviour/removes_child.rb:10:in `on_envelope'
 | /home/foreman/gems/gems/concurrent-ruby-edge-0.2.4/lib/concurrent/actor/behaviour/abstract.rb:25:in `pass'
 | /home/foreman/gems/gems/concurrent-ruby-edge-0.2.4/lib/concurrent/actor/behaviour/sets_results.rb:14:in `on_envelope'
 | /home/foreman/gems/gems/concurrent-ruby-edge-0.2.4/lib/concurrent/actor/core.rb:161:in `process_envelope'
 | /home/foreman/gems/gems/concurrent-ruby-edge-0.2.4/lib/concurrent/actor/core.rb:95:in `block in on_envelope'
 | /home/foreman/gems/gems/concurrent-ruby-edge-0.2.4/lib/concurrent/actor/core.rb:118:in `block (2 levels) in schedule_execution'
 | /home/foreman/gems/gems/concurrent-ruby-1.0.5/lib/concurrent/synchronization/mri_lockable_object.rb:38:in `block in synchronize'
 | /home/foreman/gems/gems/concurrent-ruby-1.0.5/lib/concurrent/synchronization/mri_lockable_object.rb:38:in `synchronize'
 | /home/foreman/gems/gems/concurrent-ruby-1.0.5/lib/concurrent/synchronization/mri_lockable_object.rb:38:in `synchronize'
 | /home/foreman/gems/gems/concurrent-ruby-edge-0.2.4/lib/concurrent/actor/core.rb:115:in `block in schedule_execution'
 | /home/foreman/gems/gems/concurrent-ruby-1.0.5/lib/concurrent/executor/serialized_execution.rb:18:in `call'
 | /home/foreman/gems/gems/concurrent-ruby-1.0.5/lib/concurrent/executor/serialized_execution.rb:96:in `work'
 | /home/foreman/gems/gems/concurrent-ruby-1.0.5/lib/concurrent/executor/serialized_execution.rb:77:in `block in call_job'
 | /home/foreman/gems/gems/concurrent-ruby-1.0.5/lib/concurrent/executor/ruby_thread_pool_executor.rb:348:in `run_task'
 | /home/foreman/gems/gems/concurrent-ruby-1.0.5/lib/concurrent/executor/ruby_thread_pool_executor.rb:337:in `block (3 levels) in create_worker'
 | /home/foreman/gems/gems/concurrent-ruby-1.0.5/lib/concurrent/executor/ruby_thread_pool_executor.rb:320:in `loop'
 | /home/foreman/gems/gems/concurrent-ruby-1.0.5/lib/concurrent/executor/ruby_thread_pool_executor.rb:320:in `block (2 levels) in create_worker'
 | /home/foreman/gems/gems/concurrent-ruby-1.0.5/lib/concurrent/executor/ruby_thread_pool_executor.rb:319:in `catch'
 | /home/foreman/gems/gems/concurrent-ruby-1.0.5/lib/concurrent/executor/ruby_thread_pool_executor.rb:319:in `block in create_worker'
 | /home/foreman/gems/gems/logging-2.2.2/lib/logging/diagnostic_context.rb:474:in `block in create_with_logging_context'