Notifications stopped appearing yet config was not changed. notify.log was empty

CMK version: 2.0.0p23
OS version: RHEL 8.6

The notification log file was empty: /omd/sites/Nagios_elwi/var/log/notify.log. notify.log.1 was last updated Nov 22 11:19. (about 50 hours previously) Check MK and its server had been up for 7 days.
Cycling omd made no difference. Neither did rebooting the Check MK server.
On the Check MK Master control menu, I toggled Notifications off, then on. This appeared to work as later notifications were emailed and notify.log shows their processing.
Is there a known bug or does anyone have experience resolving this?