Hello Richard, thank you for your immediate answer.
Unfortunately, rerunning sfc does not change anything
and I can't repair install windows (I tried it years ago and it ended up in an error).
I was hoping for some less drastic solution then reinstalling OS, because this would take a lot of time and work.
However, I checked the diagnostic diary box and got the following error number and description:
Error 815: The broadband neetwork connection could not be established on your computer because the remote server is not responding. This could be caused by an invalid value for the 'service name' field for this connection. Please contact your internet service provider and inquire about the correct value for this field and update it in the connection properties.
I will try that tomorrow. But since ubuntu uses the same connection (I didn't have to install any new connection, when I ran it) - I am not sure if this is really the cause ...