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Laurentiu - my guess is that data in the configuration directory has become corrupted, you could try this:
- backup your library and configuration data, see ==>> How do I backup calibre
- run the Welcome Wizard (Preferences bottom right corner).
- on Windows that will reset the configuration data back to the defaults (you'll lose your preferences changes, optional plugins etc) but it will leave the current library untouched.
- I'd be surprised if it was different on OS/X - in part that's why I suggest a full backup first - abundant caution
Added: following on from theducks post - if there is a gui.py in the configuration directory (not having a Mac I don't know that there is), then when calibre is NOT running, rename it something like gui.py.save and start calibre - that will create a fresh gui.py, you may lose a few preferences. That would work on Windows, but lots of things work on Windows but not on OS/X and vice versa of course.
BR