Windows Server 2019 Backup Stopped working

Since both the physical server and VM started failing when backing up to the same NAS, I would investigate the backup destination and existing backup set before reinstalling Windows Server Backup again.

The error:

0x8078014D

There was a failure in updating the backup for deleted items

suggests the failure occurs while Windows Server Backup is updating the existing backup data.

As a clean test, create a completely new NAS share/subfolder and run one backup to that new location. Microsoft documents that backups written to the same remote share can replace the previous backup, which is another reason to separate backup destinations carefully.

I would then check:

vssadmin list writers

and review:

Event Viewer

→ Applications and Services Logs

→ Microsoft

→ Windows

→ Backup

→ Operational

Also check Synology logs for SMB errors, quotas, snapshots, file locking or storage errors.

If the backup succeeds immediately to a new share, the old backup set/share is the likely problem. If the new share also fails, investigate VSS and SMB connectivity next.

I would not delete the old backup set until you've confirmed you no longer need it for recovery.