Time Machine couldn’t complete the backup to “DiskStation.local” (synology ds231+)

The backup disk is not available. Make sure the backup disk is connected or select a different backup disk.

I've been backup to the NAS for years. Currently on

Big Sur 11.7.1 &

DSM 6.2.4-25556 Update 6 on NAS

Until late October 2022, it fails with the above message. I did verified that I am following synology's kb all along

• How do I back up files from my Mac to Synology NAS using Time Machine?
• What should I do to fix the problem when connecting to Synology NAS using Time Machine?

Tried AFP only, SMB only, AFP & SMB enabled. Changed network cable. No good.

However I can mount the NAS shared folder and copy to and from it without issues.

From macos console

default 10:12:13.284113+1300 backupd GetNetAuthAuthAgentPort: Calling bootstrap_look_up3() for com.apple.netauth.user.auth; uid = 0, euid = 0

default 10:12:13.284519+1300 backupd Calling bootstrap_look_up3() for com.apple.netauth.sys.auth

default 10:12:13.284830+1300 backupd GetNetAuthAuthAgentPort: gNetAuthAuthAgentPort = 14915

default 10:12:16.002693+1300 backupd Connecting to hdiejectd

default 10:12:16.013964+1300 backupd Connected successfully

default 10:12:47.270067+1300 backupd Connecting to hdiejectd

default 10:12:47.271300+1300 backupd Connected successfully

error 10:13:12.956345+1300 backupd-helper Invalid mountpoint '/Volumes/Backups of MBP' - failed to open '/Volumes/Backups of MBP/.iotest', error: 5 Input/output error

error 10:13:13.277782+1300 backupd Invalid mountpoint '/Volumes/Backups of MBP' - failed to open '/Volumes/Backups of MBP/.iotest', error: 5 Input/output error

error 10:13:13.677990+1300 backupd Invalid mountpoint '/Volumes/Backups of MBP' - failed to open '/Volumes/Backups of MBP/.iotest', error: 5 Input/output error

error 10:13:13.880742+1300 backupd Backup failed (19: BACKUP_FAILED_TARGETVOL_NOT_MOUNTED - The backup disk could not be resolved, or there was a problem mounting it.)

error 10:13:14.117310+1300 backupd Diskmanagement failed to unmount disk '/Volumes/Backups of MBP', error: {

Action = Unmount;

Dissenter = 1;

DissenterPID = 0;

DissenterStatus = 49168;

Target = "file:///Volumes/Backups%20of%20MBP/";

}

error 10:13:14.389942+1300 backupd Diskmanagement failed to unmount disk '/Volumes/.timemachine/DiskStation._smb._tcp.local./B2B1C85B-A80A-42D7-8AD5-0634C2F8BA8D/NetDrive', error: {

Action = Unmount;

Dissenter = 1;

DissenterPID = 17893;

DissenterPPID = 1;

DissenterStatus = 49168;

Target = "file:///Volumes/.timemachine/DiskStation._smb._tcp.local./B2B1C85B-A80A-42D7-8AD5-0634C2F8BA8D/NetDrive/";

}

error 10:13:14.393515+1300 backupd Failed to unmount '/Volumes/.timemachine/DiskStation._smb._tcp.local./B2B1C85B-A80A-42D7-8AD5-0634C2F8BA8D/NetDrive', error: Error Domain=com.apple.diskmanagement Code=0 "No error" UserInfo={NSDebugDescription=No error, NSLocalizedDescription=No Error.}

Any ideas on whether it's software settings or hardware issues? All my 3 Macs are unable to backup to NAS. All used to be able to backup via Time Machine and they are all on 11.7.1

Cheers,

Mitch