Ok after some more testing I found the cause of the problem: It seems like a the subscription gets an exception whenever I use a "force" mode to write a value on my plc. When this happend I need to restart the plc and opcua4unity can subscribe as it should be. I guess the problem is within the plc system then, so I reported it to the guys @CodeSys. 

Thank you!

Hmm, just updated to Unity 2019.1.1f1 and the exception is gone. Althoug, downgrading to 2018 did not reproduce it either. So, I guess it was a project configuration problem after all.

Thank you for your help!

Thanks for the quick response!

Yes IL2CPP, Unity 2018.3.12f1 (due to other dependcies) and .Net20.

Let me add some other network stuff to the app to check if networking is ok!