Ok, I think I have done all obvious things like deleteing cookies, cache, restarting apache, updating debian to the latest revision, restarting server, even testing on another server. I still have no entry in the apache errorlog, only the posted entry in the acceslog. Suprisingly IE and chrome give me the errormessage on the screen whereas Firefox shows me a blank screen. No errors in Javascript which I am able to find. So I will wait paitiently for the ongoing development.
Thanks
P.S. Is there an easy way to go back to a previous checkout by using svn merge -r x:y? svn-helper seems not to work for me.