Since vbgallery does not come in a german language what is that specific message say?
Memmelsdorf - Brauerei Gasthof Höhn - Lagerkeller Galerien zur Tour-de-Bier
That link works so basically from what I see we are back to this. Remove this from showimage.php and the link without c=5 should work as that is what has worked for quite a few people. I would suspect the issue is because of a vbulletin hack as I am unable to get any issues on a clean
vb and vbgallery install with or without the catid present.
http://www.photopost.com/forum/1235308-post2.html
I am assuming your error message says something about some search and thats because somewhere in the code because of a hack or something the program thinks cat is being set to searchterms.