I have been having bizarre memory problems as well lately.
First unattended-upgrades tried to install some updates for symbiosis-email and symbiosis-email-activesync on Friday. For some reason the upgrades required a reboot, which was not possible because something went completely wrong on the VM and I was unable to do anything but execute a hard reset because something was hogging all the memory and cpu.
After the reset everything seemed back to normal. unattended-upgrades was actually a complete surprise for me as I had not realized it was installed. I actually thought I had checked that it wasn't installed, but apparently I was wrong. After this I am even more against it and am considering removing or disabling it.
Tonight I got some "Cannot allocate memory" errors from the firewall and symbiosis-monit scripts. I don't know what was causing it, but apparently something had started up clamav-daemon again.
I had previously tried disabling ClamAV and SpamAssassin, first by removing all config/antivirus and config/antispam files (which didn't seem to prevent it from running) and later by disabling the services in systemd. This seemed to have worked until tonight.
Now I stopped and disabled clamav-daemon.service again and got a message that the service might be started again by clamav-daemon.socket, so I stopped and disabled that too. Removing ClamAV is not possible since Symbiosis depends on it.
IMHO ClamAV and SpamAssassin are completely inappropriate for smaller VM's and should be optional when installing Symbiosis.