OK, more searching solved the issue, which can be found here: http://support.microsoft.com/kb/975723 My toolbar folders were corrupt. Everything is working again without toolbar customization. But when I start modifying the toolbars again (I need a set of 20 buttons or so to be really fast an efficient in PowerPoint), the whole saga starts again. I will keep you posted about my experience with Microsoft PowerPoint 2011.
I have been battling with PowerPoint 2011 for the Mac for the past hour and it seems seriously flawed. When entering slideshow mode, it just crashes. Searching online for a solution reveals dozens of forum discussions about the same issue that are unresolved. Do not upgrade from Office 2008. Repeat, do not buy it, it is not stable yet!
Usually I am an early adopter of software and can live with a few bugs here and there. Not being able to go into slideshow mode kills the purpose of PowerPoint, this is a serious flaw.