Same problem here. I’m trying to find the cause of the issue, but it’s very hard, because it has something to do with CommonActionCommited_PythonCallable, which is something of the PythonScript plugin and has nothing to do with my code. 
EDIT:
After some furious debugging (I wasted a lot of time like you) I found the issue 
. Because of the loading phases of the two plugins, the variable GIsDuplicatingClassForReinstancing was true and the Python plugin was creating a default object while that variable was true. This is what I did: I edited PythonScriptPlugin.uplugin and changed LoadingPhase of PythonScriptPlugin from PreDefault to Default. I don’t know if this breaks something inside the PythonScriptPlugin though, but I don’t use it so I don’t care (and I couldn’t disable it because it’s used by Niagara). I think this is an acceptable fix until Epic fixes the problem. Another fix that you can do is disabling the ensure condition in UObjectGlobals.cpp, but maybe it’s more destructive than changing the loading phase of the Python plugin.