EmonPi stops logging data


I have an emonPi and serveral emonTxs. Several times over the last month (including today) data stopped appearing in the graphs, from both the pi itself, and the remote Txs.

A reboot of the Pi seemed to have restored logging from both the local CTs and the remote (emonTx) ones.

Is there any log on the Pi that I can check, or services that should be running? I could not see anything unusual in syslog or messages (before rebooting).

The emonhub.log is usually a good source of info to start with.

This lack of logging has happened again and I saved all the logs before rebooting this time.
emonhub.log had lots of this:
2017-07-20 12:17:20,859 WARNING MainThread RFM2Pi thread is dead
2017-07-20 12:17:20,860 WARNING MainThread MQTT thread is dead

Which certainly correlates with data not being logged from various Tx units, or send on to emoncms.org

Is there other logs I should look at? I was just going to check them all at around the time the logging stopped and see if there was any helpful info.