Hi Guys,

sorry for the late reply, I was on vacancies.

Thanks for your support all of you!

@SeijiSensei: I did as you suggested. Trying to start the VM now I get (at about 98%):
"For the VM windows7 a new session couldn't be opened (my translation)
Für die virtuelle Maschine Windows7 konnte keine neue Sitzung eröffnet werden.
No error info.
Fehlercode:NS_ERROR_CALL_FAILED (0x800706BE)
KomponenterogressProxy
Interface:IProgress {c20238e4-3221-4d3f-8891-81ce92d9f913}

@JKyleOKC: I definitely made several reboots in the meantime. I installed AVM Antivirus first, then rebooted, then installed MS Office, rebooted etc.

Any further ideas?