Where exactly on the site do you get that message?
Some areas of the website are restricted to administrators, moderators, team members or people with certain faction privileges, maybe that's the cause of it? After all, you were able to post on the forums while being logged in.
If what Wilhelm von Ismay says is true, and this happened around the time of the daily backup, there is nothing you can do about it but wait it out. Everyone gets the same error at that time, but once the backup completes everything works as normal again. Shouldn't last more than about 10 minutes.