Error 500 on creating a new wiki or new wiki page

Running GitLab EE 8.7.0 (source distribution) with MySQLdb on Ubuntu14 LTS here.

While trying to create a new wiki or wiki page for a project I get an error 500. Using the git mechanism for creating pages (checking out the wiki repository / creating a file / commit & push this file) works fine so far.

Unluckily there's no error message on any logfile that give me more clues what's wrong.