As reported I’m in contact with Parallels Desktop Support. The next mail arrived tonight and I got asked to re-install Parallels completely. I did as requested. During the re-installation I got asked for my license key. So I typed it in, but Parallels Desktop told me this license key is not valid. I continued using a trial key.

After re-installing Parallels Tools in my Virtual Machine, which has not been deleted during re-install of Parallels the virtual Machine had to reboot. So I started Outlook and my issue seems to be solved. Thank yo for your support, Parallels Team!

My only remaining issue is the non-valid license Key, but I’m pretty sure Parallels is going to solve this as well.