I finally managed to update all and everything to WordPress 3.5.
I have got a bigger search issue, because I have installed a relevance search, which influenced my backend search.
This I solved with the (!isAdmin) condition.
But the search in the new mediacenter doesn't work and after trying hours to find out, I now give up and ask the community ;-).
If sombebody got the same issue and found a solution, I would be very very grateful.