Also having that problem, tried rebuilding the plugin from source and no luck (4.27)
Assertion failed: NewOperatorInputA [File:/home/anon/GIT/UnrealEngine/Engine/Source/Editor/BlueprintGraph/Private/K2Node_CommutativeAssociativeBinaryOperator.cpp] [Line: 293]
EDIT:
Wait, it works just fine now, after a tiny bit of tweaking and recompiling: