We can confirm that SSL certificate expiration reminder e-mails are being sent again. The problem is resolved.

UptimeRobot has provided us with a new API option which we have implemented within 2 hours after receiving the information. We hope this will fix the issue and are monitoring the situation.

Currently, the SSL certificate reminders do not work and unfortunately, there's nothing we can do about that. UptimeRobot has changed their defaults and there is no API yet to change those values which means it is impossible for us to synchronize your settings with UptimeRobot. We are in touch with their technical support and are trying to resolve the issue asap.