Thanks all for your kind replies. I installed the Avast English language pack to see what the correct wording in Bahavior Shield Main Settings is:

If I untick “Monitor the system for unauthoriszed modifications” and leave evrythuing else on it seems to work. Up till now no problems.

I also tried to put Retrospect folder in the in the Avast exclusion lsit, but that did not help either.

I have a funny feeling though, because before I came across this avast issue, I had working Retrospect backups, i.e. without Shadow Copy complaints; sometimes after a reboot. So for now I keep the setting unticked and will monitor over the next days (auto backups daile at 11:30 pm) and report back.

Later
Kobe