Ciao,
I have installed several websites and have always set the Sef Url Active. I have never had problems with Ajax or url no-sef like paypall and components that don't support url sef having Ajax.
I tried to contact the host and he told me that the problem is with the template.
I tried to contact the person who created the Ajax component.
And he also told me to contact you.
I used the native Cassiopeia template, to test the problem and it doesn't actually show up.
The problem occurs on the OS Services Booking component, which uses Ajax.
In the past I had it on Joomla 3 with your JA Brickstore template, neither the component itself nor virtuemart gave me this problem.
If you try to go to this url and book a Tasting. When you try to pay with Paypall you are sent to the Home page, and the operation obviously fails.
https://www.vinonobile.it/en/experience/category-visit-and-tasting
Please help me is very important to my company. The season has started and Covid has already got us in trouble in recent years
Best regards
See you soon
Luca