Found the reason: http://support.microsoft.com/kb/238404/en-us
This method of retrieving the Outlook version creates a new item to
get the version. Unfortunately, the delete call doesn't work (error
that is ignored). But even with a working deletion, the folder
"deleted objects" grows after each initialisation. So this way of
getting the outlook version is not usable. I changed it by processing
the result of Application.Version.