This is an obscure problem and we don't know what causes it. As far as we know, it only happens to people using Microsoft Internet Explorer (IE) Version 6 and older.
We have not seen the problem since we upgraded (it's free) to Version 7.
It has something to do with the cookies IE writes to your hard drive to keep track of whether you are logged in or out.
We have always been able to solve the problem by deleting all of our cookies, but we can't advise you to do this. We say this because you may NOT want to delete all of your cookies used for other purposes.
IF you want to delete all of your cookies, you can do it in IE by clicking on Tools, then Internet Options, then Delete Cookies.
Alternatively, the problem usually just goes away by itself in a day or two.
If you have any questions, send us an e-mail and we'll be glad to help.