No confirmation email?

Hello and help!
I noticed today that I don't receive confirmation emails anymore. The last email was sent April 20th.
There is another thread with the problem (https://premium.wpmudev.org/forums/topic/confirmation-email-not-working) - we don't have gCal enabled on our site, though.
I tried to resend the confirmation email but that didn't word either.
Appointments+ Version is 1.8.2