I have recently installed Protégé 4.1 and plugins for it. And today it has crashed when I tried to create new ontology. Error message: “NoClassDefFoundError: org/protege/editor/owl/ui/action/ProtegeOWLAction”.

Active googling gives only one possible solution:  “It looks like your Protege 4 installation is corrupted. I suggest that you remove the old installation, and make a clean install from the website.”

But I have found that it can be repaired less radical way. Just look at Protégé’s console window and read plugin loading error messages. Then delete broken plugins.