Logs show this error:

Too deeply nested.
 (ActionView::Template::Error)
/usr/local/rvm/gems/ruby-2.0.0-p353@test_develop_pr_core-1/gems/rabl-0.9.0/lib/rabl/engine.rb:240:in `dump'
/usr/local/rvm/gems/ruby-2.0.0-p353@test_develop_pr_core-1/gems/rabl-0.9.0/lib/rabl/engine.rb:240:in `format_json'
/usr/local/rvm/gems/ruby-2.0.0-p353@test_develop_pr_core-1/gems/rabl-0.9.0/lib/rabl/engine.rb:64:in `to_json'
/usr/local/rvm/gems/ruby-2.0.0-p353@test_develop_pr_core-1/gems/rabl-0.9.0/lib/rabl/engine.rb:39:in `block in render'
/usr/local/rvm/gems/ruby-2.0.0-p353@test_develop_pr_core-1/gems/rabl-0.9.0/lib/rabl/engine.rb:285:in `cache_results'
/usr/local/rvm/gems/ruby-2.0.0-p353@test_develop_pr_core-1/gems/rabl-0.9.0/lib/rabl/engine.rb:39:in `render'
/var/lib/workspace/workspace/test_develop_pr_core/database/postgresql/puppet/3.0/ruby/2.0.0/app/views/api/v1/users/create.json.rabl:2:in `_app_views    api_v_users_create_json_rabl___4493772589656306360_147762980'

Seems to be due to oj 2.10.3 which started causing the error. Release notes mention "Rational recursive loop caused by active support fixed".