The filename of the image had special characters in it (ü) .
What happened was that the filename was changed to regular characters (u instead of ü), and the text in the popup code lacked the characters, they were just left away.
So,
- Original (local) filename was:
rückbau-düsseldorf-5.jpg - After adding the popup image, (uploading from local computer) it changed to:
ruckbau-dusseldorf-5.jpg (Which is correct IMHO) - The popup text showed:
[[popup file="2012-06/rckbau-dsseldorf-5.jpg" description="(thumbnail)" alt="Rückbau Düsseldorf 5" align="inline" ]] - The used character set in the templates is utf-8
Of course I know that I can work around it by changing the filename in advance, but since this can also point to something which goes wrong, I decided to ask it here anyway.