There was no error message. I started downloading a bunch of mails and then Outlook closed automatically!
After a restart of my entire PC (Windows), Outlook didn't show the reading pane. I tried to reinstall Outlook, but there was no changing. The reading pane is always off. I can't activate it!

I had that problem about 6 months ago yet. I found a solution that solved the problem, but I cannot remember! So I know, that it is a common problem of Outlook 2007!

/ Sven