After invesrtigating some frustating time on this problem i figured out a solution thanks to an old thread:
It seems very important to give every item a label. I just did that with my countless items just to give it a shot.
In Openhab 2 with textual files it was more intuitive to give every item a label especially with the use ofe BASIC UI. But in Openhab 3 over the UI it is easily left empty because there is not much need for it.
To my amazement that solved my issues with this “Check connection error”. Hope this will solves also for others these frustating problem/bug.