Sorry to necrobump, but I'm having a similar issue. I too use Picard on OSX, and I can save files with it but I can no longer access the Preferences menu (or the "About MusicBrainz Picard" function, so I cannot tell which version I am running).
Here's the output from "Show Error/Debug Log":
W: 02:18:11 Plugin directory '/Applications/MusicBrainz Picard.app/Contents/Resources/plugins' doesn't exist
E: 02:18:11 Plugin 'coverart' : Traceback (most recent call last):
File "picard/plugin.pyo", line 292, in load_plugin
File "/Users/[name]/.config/MusicBrainz/Picard/plugins/coverart.py", line 36, in
ImportError: cannot import name mimetype