One of the reasons that can cause this is the absence of FARMS key under
Sometime it may happen that the key is not present and will cause the backups to fail. The significance of this key is that it contains the information about the SharePoint farm.
This Key is created on SQL Backend Servers and contains the names of frontend servers that put their data on this backend server.
If you encounter such issue, you can use the following command to create the key and reattempt the backup.
bpresolver.exe query -c svw8scr5 -ct 8 -init_beds "Microsoft SharePoint Resources:\*"
Recently, while working on one of the cases, I used this command and it resolve the backup failure for us.