blank page after log in (whole site)

I've got a blank page after I log in with my user account (Xander84) on the whole website including the forums.
It seems the server returns nothing, when I delete my cookies everything works again, but I can't use the user forums like this obviously.
I've tested with multiple browsers on PC and mobile devices and always the same, so I assume it is a server bug (maybe only in my profile) or a problem with cookies?