Means the problem is in one of the files in the config folder.
My initial guess is gui.json.
With calibre closed, copy the saved config folder back to where it belongs and rename gui.json to something like zave-gui.json, then restart calibre - calibre will create a fresh gui.json.
If it works you can use the zave-gui.json to help reinstate lost settings. I wouldn't copy anything from it, just use it as an information source to reinstate settings manually.
If it doesn't
BR
You owe me ten minutes of ODI World Cup final viewing
