May, I found the reason. Although I did the same procedure of installation, this time, the authorizon for battery optimization seemd to be a (the one) reason.
After manual change of battery optimization (->forbidden), the connection was established well. Never did this in the first hour after installation :-)
Maybe it's a useful example for some further unhappy users
After manual change of battery optimization (->forbidden), the connection was established well. Never did this in the first hour after installation :-)
Maybe it's a useful example for some further unhappy users