1. You have Hyper-V 2019 environment with Acronis Backup Agent for Hyper-V installed,
2. Backup finishes with the following warning
'Windows error: (0x8000FFFF) Catastrophic failure'
Issue in the product.
This issue has been fixed in Update 4, please update to the latest build.
For earlier builds, this warning can be safely ignored as it does not affect the created backup.