First problem is the /var/log is full. quick fix is to delete daemon.log (it will come back).
Second (and unrelated actually), is this a 4Gb SD Card?
[edit]
What I then meant to say is that it could be that as it cannot write to disk rsyslog may be keeping the logs in RAM. If you make some space then restart the service sudo systemctl restart rsyslog.service does RAM useage drop?
Keep an eye on /var/log and see if the RAM use returns when it fills up again.
I mention a change to the rsyslog.conf file to stop the daemon.log file filling up here emonSD next steps: log2ram - #8 by borpin