Looks like rdsys backend crashed: https://gitlab.torproject.org/tpo/anti-censorship/rdsys/-/issues/222
It is back after a restart, but I'm still investigating the source of the problem.
Quoting alertmanager@hetzner-nbg1-02.torproject.org (2024-09-02 01:43:45)
Total firing alerts: 1 Total resolved alerts: 0
## Firing Alerts
Time: 2024-09-01 23:43:14.934 +0000 UTC Summary: There might be a problem with the email delivery Description: No email has arrived to gettor for the last 6 hours
##Resolved Alerts