I changed a lot of the plugin files in the last view weeks (e.g. functions.php, header.php, loop-index.php, etc.). Therefore I did not update the entire brightbox folder, but only the files listed in the file version_brightbox.rtf delivered with the new brightbox 1.2.
After the update, the Main Menu looks like on this screenshot: http://www.shutter-lag.com/menu.png.
I therefore downgraded only the framework folder again and it goes back to normal.
Does anybody see this bug as well, or is that due to my changes in the theme?
Update: I renamed my old theme folder and installed the entire version 1.2. Works now...So I guess I will start looking for all the files I changed ;-)