After a long time of debugging the annoying error that popped up in my firefox error console:
Error: Unrecognized Syntax (...
I finally managed to trace it down! Turns out that jQuery 1.2.6 still allowed the selector syntax [@, which starting from jQuery 1.3 was removed. After figuring that out I grepped through all modules and was able to blame lightbox2! Yes, lightbox2! For some reason the current official version has not fixed those, but after downloading the dev version the dreaded error was gone.