I helped another guy with a solution who came accross this bug.
The maps are requested by an ajax post. The maps will get returned in a XML format. This format is only accepting ANSI.
There is a common map (which is also in your image) RocketForest which has the name "RöcketForest". The character ö is not ANSI so the XML cannot be parsed (at least for Firefox).
You can goto to the following page. This is requesting the maps for the gametype VehicleCTF.
You might see an XML error containing some information what file could be corrupt.
For this specific error, you have to change RöcketForest to RocketForest in its ini file "VCTF-RocketForst.ini" resulting like this:
NumPlayers=8 to 20 Players Author: scarface