Format problem #<ActionController::MimeResponds::Collector>

[Jellyfrog]

Getting this for each page-view in the log;

Started GET "/rb/server_variables.js" for XXX at 2015-08-24 11:11:15 +0200
Processing by RbServerVariablesController#index as JS
  Current user: ZZZ (id=45)
Format problem #<ActionController::MimeResponds::Collector:0x000000089a76c8>
  Rendered plugins/redmine_backlogs/app/views/rb_server_variables/show.js.erb (6.8ms)
Completed 200 OK in 15ms (Views: 8.0ms | ActiveRecord: 0.4ms)

Redmine version                3.1.0.stable
Ruby version                   2.2.2-p95 (2015-04-13) [x86_64-linux]
Rails version                  4.2.3