This is possibly a related question.
I have a user who has been e-mailing me for over a week about not being able to access the site, either from home or work. He says it simply doesn't load. Yet, he doesn't get a "page not found" error. It's a "cannot find server" error. So far he's the only one that's reported any problem to me. I have asked him to clear his cookies and browser cache, but to no avail. I've tried giving him alternate URLs to try: nothing. I cleared all of my banned IPs (they were old anyway) but his IP doesn't seem to be listed.
One thing I did discover was when I attempted an IP trace/lookup, no information could found. It said the host didn't exist. Could that be related?
BTW, I am running the latest version of phpbb and he started experiencing these problems right after the upgrade. Again, he's the only user that has reported log-in problems to me.