The two weeks thing is actually ten days.
And as for coming back to a previous tenant for damages after the apartment has been signed off it IS possible if the damage that was found would not have been found during a normal walkthrough and is considered a "hidden defect" (versteckte Mängel). This could well be considered a hidden defect as there would be no reason to check such a panel in detail. I guess this is a difficult situation because you have no way of proving that the panel was glued by a tenant before you...
I would definitely talk to the Mieterverband, but you may well find out that you are liable.
I have attached an article that appeared in K-Tipp, a Swiss consumer magazine, which may help you understand the process better (although it is in German).
Good luck!