APFS Sparsebundle Attach Failure over SMB to Synology NAS (Code 13 / 257)

Hello,

I am reporting a reproducible Time Machine failure when backing up from an M4 iMac (latest macOS) to a Synology DS216+II NAS over SMB.

Environment:

• Mac: M4 iMac
• macOS: Latest public release
• NAS: Synology DS216+II
• DSM: Latest available version
• Protocol: SMB3
• Time Machine target: Synology shared folder (officially enabled for Time Machine)

Observed behavior:

1. Time Machine successfully creates the sparsebundle disk image on the NAS.
2. During the attach/mount phase, the process fails.
3. Logs show:
   • Permission denied (Code 13)
   • Sandbox denial (Code 257)
   • BACKUP_FAILED_DISCONNECTED_DISK_IMAGE (70)
   • APFSMachineStore structure missing during mount attempt

The failure occurs consistently at approximately the same early transfer size (~390MB).

Troubleshooting already performed:

• Reset NAS shared folder permissions and applied inheritance
• Disabled advanced permissions
• Disabled encryption during backup setup
• Cleared local Time Machine preferences
• Removed orphaned mounts under /Volumes/.timemachine
• Retested with clean sparsebundle

The sparsebundle file is successfully created over SMB, but APFS attach fails at the mount stage, followed by sandbox denial.

Question:
Is this a known compatibility limitation involving APFS sparsebundle attach over SMB with older SMB implementations, or a macOS sandbox enforcement regression?

I can provide sysdiagnose logs upon request.

Thank you.