I had the same problem. (possible cause: using AVG TuneUp registry clean-up).
Uninstalling Cura and Minecraft didn't help.
I found that it helps to make the following change to Cura\util\pymclevel\materials.py
From:
def addYamlBlocksFromFile(self, filename): if yaml is None: return
To:
def addYamlBlocksFromFile(self, filename): if True: return