@josdb - At a guess I'd say something has gone awry with the file gui.json
Open configuration folder via
Preferences->Miscellaneous and click
Open configuration directory
- Close calibre.
- Rename the file gui.json to save_gui.json
- Try starting calibre 2.20 or 21
- If it works my guess was right, otherwise delete the gui.json and rename save_gui.json back to gui.json
Let us know what happens - if my guess was wrong someone else will have some other suggestions.
BR