Module Cache 500 error

Hi all,

Occasionally I am getting this 500 error: [Module:Cache]handlerProcess return 500 due to cant alloc memory.

Any idea what's causing it? Sever is Ubuntu 20.04.1, runnig OpenLiteSpeed 1.7.12 and CyberPanel 2.1

I'm running monit to monitor ram and swap usage and it didn't seem to be running out of memory:
Swap usage limit If greater than 25.0% then alert
Memory usage limit If greater than 75.0% then alert

I increased the php memory_limit to 256M, though I don't think it's related to the issue.
I'm still getting the error in the logs after the memory_limit change.

Any other ideas?