One of my admins can't access the admin page, the only thing that shows is "'" and absolutely nothing else. The source code is perfectly normal except that there is a "'" at the very beginning, which really shouldn't be there.

He has also had problems with getting in to our admin forum, which was fixed when I demoted and gave him admin privs again. From what I can see group 1 is among his groups, and his status is Administrator. It also works just fine if I become his user. He says this happens regardless of how many times he removes his cookies, he has even made a complete reinstall of his system between trying. He isn't using a firewall or anything like that either.

Has anyone seen this happening? Any ideas?

I haven't seen this before... which version are you running?

Does his username contain any special characters?

Oh, sorry. It is 6.1.0 with all the released bug fixes, and a lot of hacks.

Nope, there is nothing strange with the user at all that I can see.

If you can login as him..... and it works.... then it must be a cookie thing. Otherwise you'd experience the same troubles too.

But you ruled out all the common culprits (firewall etc..)

Not sure.

Yeah, that's what is so confusing...