Appointments+ conflict after latest upgrade

Hello there. Back from holidays, I have updated my websites.
But after all upgrades, the backend became unstable, and some things were not working.
Very slowly and with FTP I did manage to deactivate all plugins, and figured out that the one causing all the problems was the Appointements+.
Now, if I activate it, I'm not able to see the widget placeholders in the Appearance -> Widgets area of the WP backend.

Any idea on the solution? I pretty like this plugin, and it's a pity to have it deactivated...

Many thanks,
Francesco