Your comments

It actually works if I specify an endpoint at the OPC UA server:

So, instead of opc.tcp://localhost:4840/ I specified opc.tcp://localhost:4840/Test.
So far this is ok for me.

No. I get the error message "NullReferenceException".

Unfortunately it is the same. However all other servers connect. In fact it seems they ignore the path after the port somehow:

I will try with less variables.

All my Clients work: OpcWatch, UE Expert and even OPC UA nodes of node-red. It is all localhost. 

Thanks for your help! :-)

Thanks for the hint. Yes it is only visible in edit mode. However it does not allow me to delete it as it is a child of a Prefab instance. I've now set it to hidden.