Solution:
Hello Romain, and welcome to StackOverflow!
It appears that the new theme you activated is conflicting with your plugin “wp-booking-calendar.” This usually happens when two plugins or a plugin and a theme use the same class or function name.
In your case, the error:
Cannot declare class wp_booking_calendar_public_setting
indicates that the class wp_booking_calendar_public_setting is already declared elsewhere—likely in your theme or another plugin. Normally, plugins avoid such conflicts by using unique class names, but in this instance, a naming collision occurred.
Next steps:
Share the full name of your theme.
I can review the theme’s code and identify where the class conflict occurs, and suggest a fix.