Sounds like a javascript bug.  I have not seen this, so I'm guessing it's an IE8 issue. I will need to get set up to test in IE8.

In the meantime, you might be able to help with debugging.  Do you see a little error icon at the bottom of the screen when this happens?  If so, does clicking it tell you anything?

no error message is shown

  --Philipp.....Tue Apr 03 15:43:31 +0000 2012

Also, when in IE 8, when I go to the "options" tab, it does not show the card-menu anymore

  --Philipp.....Tue Apr 03 15:44:09 +0000 2012