Appointment booking is not completed successfully

[pax77-dd]

Hello,
The appointment booking has not been working for some time, I can't remember when. I suspect that it is due to an update of nextcloud or the app, as I installed the app some time ago and it was stable.
Here is the error description:
After filling out the form fields, checking the GDPR box, the result is an error message; “An error has occurred. Please try again later, thank you.” Previously, the information page for e-mail confirmation of the appointment appeared in this process step.
How can I fix this problem?
Kind regards
Remo

[SergeyMosin]

Can you check Nextcloud logs ? Are there any errors related to appointments ?

Also, usually an error URL look like this https://your.domain.com/apps/appointments/pub/d1A0PL_xSItmGXjB0A2/form?sts=2&d=... , what is the digit after form?sts= in your error page URL ?