I've seen that only a couple of times and each time it was related to an
Outlook COM addin that was doing something that prevented starting Outlook
that way from a standalone program when Outlook was already running.
Thanks for your reply. I run skype, but there seems no add-in in outlook
from skype.

I only found one add-in ("properties-pages of the exchange-extension"), but
disabling it wouldn't help unfortunatly...

Any other ideas?