So I will make two unit tests, one for ComplexObsResource1_10 and one for ComplexObsResource2_0. The tests will just show that editing works with the former resource and fails with the latter resource (by hopefully generating the same APIException as mentioned above in this thread).
Hopefully this will help me figuring out where the change from 1.11.x to 2.x kicks in that breaks the behaviour of my custom resource and handlers.
However, I’m already struggling with step 0 
, please see this post.