This looks similar to #20231 but not quiet. I had run db:seed succesfully with a later commit than 5a144e4bf516db2dff4ffb798ebeca19eccd624f)

$ git merge-base --is-ancestor 5a144e4bf516db2dff4ffb798ebeca19eccd624f a154c816 ; echo $?
0

Now forwarding to 8a6a98c0e1950ddbf1f744021f746ec8aa9e06c5 it fails again like:

2017-07-28T12:56:33  [app] [D] Setting current user thread-local variable to nil
rake aborted!
JSON::ParserError: 784: unexpected token at 'null'
/home/agx/foreman/app/services/ui_notifications/seed.rb:15:in `configure'
/home/agx/foreman/db/seeds.d/17-notification_blueprints.rb:38:in `block in <top (required)>'
/home/agx/foreman/db/seeds.d/17-notification_blueprints.rb:38:in `each'
/home/agx/foreman/db/seeds.d/17-notification_blueprints.rb:38:in `<top (required)>'
/home/agx/foreman/db/seeds.rb:43:in `block (2 levels) in <top (required)>'
/home/agx/foreman/app/models/concerns/foreman/thread_session.rb:75:in `as'
/home/agx/foreman/app/models/concerns/foreman/thread_session.rb:81:in `as_anonymous_admin'
/home/agx/foreman/db/seeds.rb:42:in `block in <top (required)>'
/home/agx/foreman/db/seeds.rb:36:in `each'
/home/agx/foreman/db/seeds.rb:36:in `<top (required)>'
/home/agx/foreman/vendor/gems/ruby/2.3.0/gems/railties-4.2.9/lib/rails/engine.rb:547:in `load_seed'
-e:1:in `<main>'
Tasks: TOP => db:seed