To make a long story short, their Firewall was not installed by design since I have no intention to ever use it. I now found out that it is needed to share drives & folders thought I see no connection. It can be disabled, but it has to be there.
It wouldn't be so bad if M$ would stop using useless, cryptic text & numbers for their error messages so one would have a remote idea where to look. Especially with the text "unknown error".