Great. I am 99.9% sure the issue is with the logs filling up.
To help with the time to fill up,
sudo nano /etc/rsyslog.conf
look for the line like this and make sure the entry daemon.none is there - if not add it.
*.*;auth,authpriv.none,daemon.none -/var/log/syslog
then
sudo systemctl restart rsyslog.service
You should now see just the daemon.log file increasing in size and syslog rising far more slowly. This should double the time between failures.
@TrystanLea @glyn.hudson - this needs looking at again. I have a suggested solution. A combination of emonSD next steps: emonhub logging (message 1) to create the log file for emonhub and emonSD next steps: emonhub logging - #13 by borpin (message 13) to rotate it. I am 2 days in of a test of this setup 
. I’d deploy 2 additional files (1 x monit, 1 x rsyslog) for the emoncms log as well.