Please confirm the following:
- The sysvol share is available on the DCs
- You can access the sysvol volume from the RMAD server using the same credentials that the backup runs as by entering the path such as: \\DCname\c$\windows\sysvol
- Check that the FRS, DFSR or replication service is up and running, then perform a backup.
- On the Domain controller open a command prompt and enter the command "vssadmin list writers". This list should contain "DFS Replication service writer". If it does not, contact Microsoft.
If access to the share and the volume works, check to ensure there are no issues with sysvol replication on the affected DCs