Error user mail is not sent / admin mail is not sent when booking

Hello,

I have configured my WP site to use WP mail SMTP as a mail service and everything seems to work as expected BUT

NOT with the Booking calendar.

I can send out a test mail from the WP mail SMTP plugin test page. When I book usingg the booking calendar I get an error message that the mail didn’t send to the admin NOR the user

Where can I troubleshoot? What should I do first to be able to see what can be the issue?
I tried to turn on the “use PHP mailer” but that didn’t work either

Any suggestions?

Thanks/ Dominicus

The page I need help with: [log in to see the link]