I just started having this issue as well.
I'm running on a KVM instance running on my own hardware.
The disk has 26.34 GB space remaining.
System memory is 78% free.
My system is stock Ubuntu 14.04 + mailinabox config.
One weird network config item to note, the NAT for ssh doesn't resolve to the mailinabox server. I have a jump server which is a completely separate device which acknowledges ssh.
Open DMARC runs fine for a period of time (days or hours, no rhyme nor reason discerned) and then I get the following error on the System Status Checks page
OpenDMARC is not running (port 8893).
I'd be happy to work with dev to determine how/why this failing and determine how to either prevent this from happening, or scripting an automated solution.