Hi,
Our security cert. was due to expire end of this month. I did just pay them to install a new one which they did yesterday. Seems like you had this problem around that same time they installed it, so maybe it was just due to installing a new one. We haven't had others with the problem, but it might be the new one being installed and a certain setting you have with your browser, like cookies etc.
Are you using IE? You might try an alternative brower, ie: use netscrape if you using IE, or vice versa and see if you get the same error with both browsers.
If you still have problems let us know, we'll see if our host can help, don't want anyone to be blocked from our site.
thanks