Remove observers.
When using observers there is always an error only in production mode:
vendor/bundle/ruby/2.3.0/gems/rails-observers-0.1.5/lib/rails/observers/action_controller/caching/sweeper.rb:3:in
`<module:Caching>': uninitialized constant ActiveRecord::Observer (NameError)
Running ECS on SLES12 with a mysql database works fine. Probably it's
the fault of the pg adapter. The newest version is 1.0, but for that you
need Rails 5.1.x. So we have to investigate when upgrading to Rails 5.
See also: https://github.com/rails/rails/pull/31671