M3 iMac Time Machine authentication fails to Synology NAS

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M3 iMac Time Machine authentication fails to Synology NAS

Hello, my M3 iMac 24" time machine copy to Synology DS225+ fails. My macOS is 15.7.7. I sent Synology Support logs and here is what they found out: "Thank you for your patience and for providing the debug logs and the results of your cross-test between your Macs.

After a thorough analysis of the tm_debug.log you provided, we has identified a specific authentication behavior that is likely causing the connection failure to the Synology NAS.

Technical Findings:

Based on the log, the NAS was able to successfully establish the SMB session and mount the shared folder, indicating that the SMB service on the NAS is functioning normally.

However, we did notice an authentication error in the log. This appears to be similar to the issue reported with macOS 26.4 and macOS 15.7.5 (and later updates), which has also been widely discussed in the Apple community.

Based on our previous investigation, NetAuthSysAgent attempts to connect using the mDNS-resolved hostname (for example, nas.local). As a result, isKnownServer returns false, preventing the system from loading the saved credentials from the System Keychain, even though the credentials exist and have the correct access control list (ACL) configuration. Consequently, NAConnectToServerSync returns error code 80, causing the authentication attempt to fail.

2026-07-11 21:41:20.865 E backupd[288:6fcff] [com.apple.TimeMachine:Mounting] NAConnectToServerSync failed with error: 80 (Authentication error) for alternate url: smb://TM_User@Strandbo_NAS.local/TM_iMac_21

2026-07-11 21:41:20.865 E backupd[288:6fcff] [com.apple.TimeMachine:General] Authentication error (80) - the correct user or password info may not exist in the System.keychain or the server may no longer allow access for this user.

Suggestion

Many users have reported that Apple resolved this issue in macOS 26.5. However, the same fix does not appear to have been included in the macOS 15.7.xbranch."

It looks like this is MacOS bug. Does anybody know if and when Apple is going to fix the problem in macOS 15.7.x?

Posted on Jul 14, 2026 1:44 AM

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Answer 1

Top-ranking reply

HWTech

Community+ 2026

Level 9

75,130 points

Jul 14, 2026 8:54 AM in response to raoul147

raoul147 wrote:

Suggestion

Many users have reported that Apple resolved this issue in macOS 26.5. However, the same fix does not appear to have been included in the macOS 15.7.xbranch."

It looks like this is MacOS bug. Does anybody know if and when Apple is going to fix the problem in macOS 15.7.x?

Since 26.5 has been out for a while now, I would highly doubt Apple will be fixing the bug with older versions of macOS. They tend to just get security updates.

I don't use TM with a NAS, so I don't know the process to set it up. If the issue is with a local DNS name, maybe if you could use a full DNS name for the device or an IP address (would need to have your router assign a static or reserved IP address for the device).

You can try clearing out the saved TM password from KeyChain/Passwords and have it save it again. Maybe an update broke something, but once reconfigured it will work again.

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Answer 2

raoul147 Author

Level 1

14 points

Jul 22, 2026 11:38 AM in response to Tesserax

Here is the answer and recommendation I got from Synology:

Thank you for your reply. I understand that despite following the previous workarounds and manual configuration steps, your Time Machine backups on macOS 15.7.7 continue to fail when connecting to your Synology DS225+.

Based on our deep analysis of the logs you provided earlier, the issue lies within the macOS SMB client’s handling of mDNS and Keychain credentials in the Sequoia 15.x branch.

To ensure you can return to a stable backup routine, we recommend the following:

Contact Apple Support: We encourage you to reach out to Apple Support and provide them with the technical findings we identified (specifically the NAConnectToServerSync error 80 and the isKnownServer returning false for mDNS hostnames). Inquire if a stability patch for SMB Time Machine is planned for a future macOS 15.x update.
Temporary Local Backup: While awaiting an OS fix, you may consider a temporary local backup to a USB drive if the data on those specific Macs is critical and cannot wait for a network fix.

We understand this is not the seamless experience you expect from Synology, and we are happy to provide any further log data or technical details should Apple Support require them to investigate your case further.

Best Regards,

Technical Support

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Answer 3

hartzell

Level 1

18 points

Aug 10, 2026 12:39 PM in response to raoul147

Hi @raoul147--

I had a similar problem (backups wouldn't work on a new TrueNAS server) and was pulling my hair out.

Finally stumbled across Dmitry Porotnikov's workaround for the bug in his post: Time Machine backup failure to Linux/Samba target - Missing kown server marker.

He even provides a little script, here's his original:

PL='/private/var/root/Library/Group Containers/group.com.apple.NetworkAuthorization.ServerMarkers/serverMarkers.plist'
sudo cp "$PL" "$PL.bak.$(date +%Y%m%d-%H%M%S)"
sudo /usr/libexec/PlistBuddy -c 'Print' "$PL"
sudo /usr/libexec/PlistBuddy -c 'Add :"192.168.88.99" bool true' "$PL" 2>/dev/null || \
sudo /usr/libexec/PlistBuddy -c 'Set :"192.168.88.99" true' "$PL"
sudo plutil -p "$PL"

In my case, I replaced the IP addresses (192.168.88.99) with the mDNS domain name my server was using (truenas.local), ran the script, rebooted, and backups started working.

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Answer 4

Courcoul

Level 6

18,880 points

Jul 20, 2026 1:06 PM in response to raoul147

On a wild guess, do either the Mac or the Synology NAS have anything but standard USASCII characters with NO spaces in their device names? You know, 0-9, a-z, A-Z, no spaces nor any symbols?

I've had connectivity issues cause my Sequoia MBP had a space in its name.

Also, make sure the Synology is updated to their latest firmware level.

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Answer 5

Tesserax

Level 10

160,780 points

Jul 14, 2026 8:46 AM in response to raoul147

Sorry, but no one here would know what Apple will or will not do about this issue. That includes even recognizing it as something that they need to resolve.

FWIW, I have a 2023 M2 Pro Mac mini that is running macOS Tahoe 26.5.2 and a Synology DS925+ NAS. I have not seen this issue running in this configuration. Actually have not seen it at all with earlier macOS releases. I don't have a standalone Mac handy that is still running macOS 15.7.7. However, I do have a Parallels VM of that macOS that I can see if I can duplicate this issue with.

If you can tell us a bit more of your network configuration, especially between your Mac and the NAS, we may be able to assist you to a solution.

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Answer 6

Zurarczurx

Level 5

5,820 points

Jul 22, 2026 11:53 AM in response to raoul147

I know this doesn't help you but I'm on 15.7.7 and my NAS backups (QNAP) are working OK. The only time I've had a problem, albeit not on Sequoia, was when I had some sort of glitch somewhere and I couldn't load the Time Machine server under its normal name (e.g. QN40TM) and instead it loaded as something like QN40TM_1. Clearing out keychain of all entries for QN40TM* fixed it for me. Like I said, couple of OS's ago on a different make of NAS so probably doesn't help you but waiting for Apple to fix this might take a long time and editing with Keychain can't make things worse.

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Answer 7

Tesserax

Level 10

160,780 points

Jul 14, 2026 9:15 AM in response to raoul147

Ok, I had an opportunity to run a TM backup from the 15.7.7 VM to my NAS. TM had no issue locating, nor authenticating to the NAS. However, the backup failed for a completely different reason than what you are seeing and I think this failure was more do to how the VM is supporting TM ... or not. I will review the system logs more throughly when I get a chance, but so far, authentication was the not the reason it failed.

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