If u r using IE6, look at the right side of ur status bar when u try to login, and there should be an icon called "privacy report", double click on it and and then u can change the security level for this site. Should be ok if u choose trust this site..........
In this case, could be a problem of IE update, there is a "repaire" function of IE, u may have a try.........Or if it just happened once, that's the problem on the server side...
I had problem to login to BBS once.(The error message was different from yours and I did not note down the message.) Anyway, you may try what I did.
From Tools-> Internet options -> delete cookies. Then it works.
If you want to keep your cookies, you may File-> Import and export ... to export your cookies to a txt file. Later, you can restore your cookies by importing it back.
Good luck!