[Zipr]

Yes, I had the same thing after upgrading to the latest version. I uninstalled and reinstalled but the problem persisted. I had to go into the config directory and delete one of the settings file. Sorry, I'm not at home to check it but the file had "gui" in it's name. You can locate the config directory through one of the buttons in Calibre's preferences section. Again, Calibre is not in front of me but I think it was in the Advanced section.

For some reason, after the upgrade, that file got corrupted or something. I had the exact same sympton as you did.

For safety's sake, I made a backup of that entire folder before I started deleting files. I suggest you do the same.

Now that I see it wasn't just me, I opened a bug ticket on it.