Ok, so that suggests an old image and explains the lack of logs following a reboot. I still wouldn’t rule out the rfm browning out just yet just because of a UPS, the rfm is far more sensitive to low voltage than a Pi, if the UPS (or any other power source) was operating at the lower end of an acceptable voltage range and dipped just momentarily, the rfm wouldn’t be able to recover. Even a good PSU/UPS can deteriorate with time and/or even temporarily with cold temps. I never rule out PSU issues until I can prove otherwise.
If it happens again please try and save any emonhub.logs, check the service with sudo systemctl status emonhub, try changing a setting in emonhub.conf and try the reset_rfm2pi script in that order and record any info, before resorting to a reboot or power cycle. Perhaps automate these steps with your alert until we get to the bottom of it. If you are near the device when it stops, can you see if the rfm2pi LED is on, off or flashes intermittently (not the pi led) the rfm2pi led is tiny so maybe familiarise yourself with it whilst things are working first.
If you pre-install the script in the thread I linked, then resetting the rfm2pi is just one command reset_rfm2pi, there’s nothing bad in there, the script’s content is in that thread so you can check it out before installing.
Can you also make sure the emonhub logging level is set to debug in emonhub.conf and also set quiet = false so we can see if there is other traffic that maybe interfering with the rfm2pi too.
I would also be keen to know what FW is installed on the rfm2pi, this is also logged to emonhub.log at startup eg
Jan 15 15:44:56 docker4 emonHub[15767]: INFO MainThread EmonHub emonHub (emon-pi variant) v2.1.5
Jan 15 15:44:56 docker4 emonHub[15767]: INFO MainThread Opening hub...
Jan 15 15:44:56 docker4 emonHub[15767]: INFO MainThread Logging level set to DEBUG
Jan 15 15:44:56 docker4 emonHub[15767]: INFO MainThread Creating EmonHubJeeInterfacer 'RFM2Pi'
Jan 15 15:44:56 docker4 emonHub[15767]: DEBUG MainThread Opening serial port: /dev/ttyAMA0 @ 38400 bits/s
Jan 15 15:44:58 docker4 emonHub[15767]: INFO MainThread RFM2Pi device firmware version: [RF12demo.13]
Jan 15 15:44:58 docker4 emonHub[15767]: INFO MainThread RFM2Pi device current settings: E i5 g210 @ 433 MHz q1
[borrowed from Hanging hardware, requiring a power cycle - #3 by Guff666]
Also Dave, there should/could be another rotated emonhub log /var/log/emonhub/emonhub.log.1 as emonhub rotates the logs at 5mb.