Sometimes some of the buttons on the toolbar are totally grey and unclickable. E.g. the "Mark all read" button. I have a temporary "solution" by right clicking the button and customizing the buttons to a different format; after that all buttons reappear properly.
Weird, if you have steps how to reproduce this, let me know. Are you using a custom windows theme?
This happens a few times a week and i am not really able to reproduce it yet. If so, i'll let u know. Since i can bypass it it is only annoying, but no show stopper for me.