I uninstalled OneDrive but it did'nt fix the problem.
Again I checked the PT Troubleshooting advices.
One point I did'nt trie was "Create a new user account".
I did so and created this new user account and it seems that there are no problems with network and PT.
So I conclude:
1.) there is no hardware problem (e.g. with networkadapter or it's driver)
2.) the problem is my current (old) user account
So far, so good. Now I have to find out what's wrong with my current user. I fear to setup everything in the new account.
