Cannot say exactly what's wrong. But if I were you then I would try to delete your Mibuso cookie on your pc. Sometimes they simply go crazy!
That happens to me too on my IE8. Using Firefox it keeps me logged in. I posted it in the feedback section there, I'm sure Luc is aware of it and is looking into fixing it.
I would suggest it's a "know issue" as there is a "Delete all board cookies" link off the board list page and this normally sorts it out for me
Yes. Often this problems comes in connection with either board or browser updates, so it matches what Daniel said - the new IE 8.0...