only small lines with fullcalendar

do you have sample link for us to see?

yes.

Link

This looks like it could be a clash caused by another plugin or your theme. To test if that’s the case, can you try with all other plugins deactivated and while running the default WordPress theme?

Does that get the calendar display working?

Yes. You are right.
When I take another theme, everything looks fine on the calendar page.

But how can I find the problem of the theme to fix it?
I couldn’t take another theme, because it will take 3 days of adjustment and with an update, I couldn’t be sure that everything looks fine…

Thank you!

The usual problem with plugin or theme clashes is loading the jQuery library incorrectly. This should help:
https://msyk.es/blog/prevent-jquery-wordpress-theme/