Underlying system is running Proxmox. From there I have the relevant two VMs: OMV and Proxmox Backup Server. The hard drives are passed into OMV as SCSI drives. I had to add them from shell as the GUI doesn’t give the option. Within OMV I have the drives in a mergerfs pool, with a shared folder via NFS that is then selected as the storage from within the Proxmox Backup Server VM. OMV has another shared folder that is used by a remote duplicati server via SSH(SFTP?), but otherwise OMV has no other shared folders or services. Duplicati/OMV have no errors. PBS/OMV worked for a couple of months before the aforementioned error cropped up.
Also possibly relevant: No other processes or services are setup to access the shared folder used by PBS.