I was having trouble loading pbUDK, which in turn revealed that pyMel itself wasn’t functioning correctly.
I discovered that it was caused by the inclusion of your pre-release userSetup.py to the scripts dir. the one bundled with UE 4.12 doesn’t have this problem.
More details: Error: 'module' object has no attribute '<some-random-thing>' · Issue #2 · cmcpasserby/pbUDK · GitHub