After researching, I found your issue seems to be similar with the experiences mentioned in below thread.
KB5001347 (April 2021 Update): Issue with Server Backup on Hyper-V Server 2016
The issue occurs when attempting the backups to a synology NAS device. At the moment, the workarounds can be summarized as below:
- Backup to a windows-based network share.
- Uninstalling KB5001347 if installed
- Seek for assistance from support engineers of Synology
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