Maybe this helps someone else.
I'm running OMV 4 as a VM under Proxmox for months without any issues.
This weekend I upgraded to kernel 4.19.0-0.bpo.6-amd64 and start having random crashes.
OMV is used for Windows File Sharing.
After returning to kernel 4.19.0-0.bpo.5-amd64 everything is back to normal.