It can mean several things. The first thing is to delete that cookie. It could be corrupt, or someone on this end could have outdated the cookie. I had this issue--on Firefxxxs, IE8, and Google Chrome. I had to find that cookie, delete it, and then re-log on. You get rid of the bad cookie (whether it is corrupt or out of date), and get a good one.
The next issue could be server trouble or forum software issues. You would have difficulty accessing the forum at all if the server is down, or you would get the error message every time you attempt to access any page. It is not impossible for a software or server issue to be having trouble signing people in. If this happens, you are probably going to have the cookie issue once it gets fixed.
If someone gets banned, you will have this issue. However, I doubt that you would be banned without being given warnings. Additionally, I doubt you would be having this error message. Rather, you would be more likely to get an "Access denied" message that has nothing to do with the host. You would be able to access all the forum pages, but not be able to post or access private areas (or sign in). However, on this site, you are not going to be banned without warning unless you commit a whopper of an offense (like posting illegal content).
If Jehovah himself or the Washtowel Slaveholdery is responsible, you would be having more than sign-in issues. The whole forum would be shut down, and you would see nothing at all except a message that the forum is down.
I think the most likely cause is the cookie issue, which for me solved the problem.