Hot backup copy failures may result from invalid synthesized backups.
This issue is exceedingly rare. To encounter this issue, one first must be using a non-standard Hyper-V deduplication block size setting.
If in the appliance master.ini file the value maxSubfileBytes_Hyper-V is not present, then the default values are being used and this issue issue does not apply.
If you are using a custom hyper-V backup block size for deduplication set using [fileDedup] maxSubfileBytes_Hyper-V, upgrade your appliance to version 10.1 and perform new Hyper-V full backups of any effected synthetic backup.