Hmm...
My colleague told me today that the same thing happen on our server, but it is a totally different setup - MDS, R77.30, Smart-1 50.
There is the same error message in logs on one of CMAs (or DMS), there are no logs from firewalls, /var/log has 10% of free space, logging is properly configured and automatically deleted. On the secondary CMA everything is fine, logs are visible. We tried to manually cleanup some more space and switch the log file, didn't help. The next thing we would like to try is just to restart this CMA. We didn't spend much time on the issue today and decided to continue tomorrow.