Here is the log entry for this corresponding call:
Started GET "/onebox?url=https%3A%2F%2Fvorkurs-discourse.cs.uni-saarland.de%2Ft%2Fbijektivitaet%2F547&refresh=false" for 188.210.56.176 at 2016-06-18 09:35:52 +0000
Processing by OneboxController#show as HTML
Parameters: {"url"=>"https://vorkurs-discourse.cs.uni-saarland.de/t/bijektivitaet/547", "refresh"=>"false"}
Rendered text template (0.0ms)
Completed 200 OK in 53ms (Views: 12.6ms | ActiveRecord: 17.3ms)
That looks fine, I think 