I've spent a half hour with Grok trying various things and nothing works. The attempt fails almost instantly with an authentication failure. This has worked for a thousand years on multiple Macs and Synology DiskStations and all of them fail the same way starting with the 26.4 update.
Confirmed this ServerMarkers workaround also fixed Time Machine → Synology authentication on macOS Sequoia.
I hit essentially the same issue on a 2019 27-inch iMac running macOS Sequoia backing up via SMB to a new Synology NAS. (Time Machine failure with a red ‘i‘ that, when clicked, indicates an authentication error)
Time Machine repeatedly failed during “Connecting to backup” with the message that there was a problem with the network username or password.
Before applying the workaround, I verified:
and:
BACKUP_FAILED_AUTHENTICATION_ERROR (29)
I then inspected:
/private/var/root/Library/Group Containers/group.com.apple.NetworkAuthorization.ServerMarkers/serverMarkers.plist
The plist existed, but my Synology hostname was missing.
I added the NAS hostname in lowercase using the PlistBuddy command posted above:
sudo /usr/libexec/PlistBuddy -c 'Add :your-nas-hostname.local bool true' '/private/var/root/Library/Group Containers/group.com.apple.NetworkAuthorization.ServerMarkers/serverMarkers.plist'
Then I rebooted the Mac.
That fixed it immediately. Time Machine progressed past “Connecting to backup,” completed the entire initial backup overnight, and this morning I manually triggered another backup of roughly 1 GB of changes, which also completed successfully.
So in my case this was definitely not an incorrect NAS username/password or Synology share-permission issue. The ServerMarkers workaround resolved the backupd authentication error.
Obviously substitute your own NAS hostname, and I would recommend inspecting/backing up the plist before modifying it.
Hopefully this helps anyone hitting the same misleading authentication error on Sequoia.
Thank you to all on here, especially Broeseli, for the help. I would have spent *much* more time on this without finding this. Oh, and ChatGPT helped a ton!
Hi there: I have the same issue like you with Sequoia (15.7.9) and TM backups, and I tried using your fix:
_JR_ wrote:
Confirmed this ServerMarkers workaround also fixed Time Machine → Synology authentication on macOS Sequoia.
... command posted above:
sudo /usr/libexec/PlistBuddy -c 'Add :your-nas-hostname.local bool true' '/private/var/root/Library/Group Containers/group.com.apple.NetworkAuthorization.ServerMarkers/serverMarkers.plist'
YES! That fixed it! I read your comment and also Broeseli's further down and it is important to enter
"your-nas-hostname.local"
instead of
"your-nas-hostname.localdomain"
as Broeseli suggests!
Thanks, the issue is fixed for now!
Best,
Magnus
Yes, you are correct. You want hostname.local, as I put in the command. Which was a slight tweak from Broeseli’s suggestion, and was the format already present in that plist. Thank you for double clarifying that.
Im glad that fixed it for you too!
Had the issue when tried to back-up to a disk connected to my Raspberry Pi running samba 2:4.17.12, the fix via serverMarkers.plist worked for me. Thanks @Broeseli!
Updated to 26.4 this afternoon. Last backup to my Synology DS223j completed less than 10 minutes ago. No problems, business as usual.
LLMs are usually GIGO (garbage in garbage out) for this sort of thing, they scrape forums like this one and find wrong solutions occasionally mixed with correct ones.
Perhaps you might have more success posting the error(s) you are seeing and the troubleshooting you've done. Have you rebooted your Mac? Can you mount the volume used for the TM backup?
Mbosse wrote:
Thanks, but I am a bit confused now: Isn't that the very command used to add a server to the plist? I'd like to remove one... Thanks!
I usually edit the plist directly. What I don’t know yet (not yet having gotten back to a Mac) is whether that particular plist is using the binary or text format for plists. Binary plists need to be converted to text and can then be edited and then converted back to binary, while text plist files can be edited directly. I don’t have PlistBuddy memorized, either.
The most useful information came from log stream --predicate 'subsystem == "com.apple.TimeMachine" OR processImagePath CONTAINS "backupd"' --info --debug:
2026-03-24 18:12:48.445652-0700 0x3fb9f Error 0x150e6a 533 0 backupd: (TimeMachine) [com.apple.TimeMachine:Mounting] NAConnectToServerSync failed with error: 80 (Authentication error) for url: smb://nsayer@chronos.ca.lan/Backups
2026-03-24 18:12:48.445748-0700 0x3fb9f Error 0x150e6a 533 0 backupd: (TimeMachine) [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.
2026-03-24 18:12:48.445805-0700 0x3fb9f Error 0x150e6a 533 0 backupd: (TimeMachine) [com.apple.TimeMachine:Bonjour] Bonjour resolution not possible for 'smb://nsayer@chronos.ca.lan/Backups'
2026-03-24 18:12:48.445833-0700 0x3fb9f Info 0x150e6a 533 0 backupd: (TimeMachine) [com.apple.TimeMachine:Mounting] Attempting to mount alternate URL 'smb://nsayer@chronos.ca.lan/Backups'
2026-03-24 18:12:48.616991-0700 0x3fb9f Error 0x150e6a 533 0 backupd: (TimeMachine) [com.apple.TimeMachine:Mounting] NAConnectToServerSync failed with error: 80 (Authentication error) for alternate url: smb://nsayer@chronos.ca.lan/Backups
2026-03-24 18:12:48.617084-0700 0x3fb9f Error 0x150e6a 533 0 backupd: (TimeMachine) [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.
2026-03-24 18:12:48.617425-0700 0x3fb9f Error 0x150e6a 533 0 backupd: (TimeMachine) [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.
2026-03-24 18:12:48.617790-0700 0x3fb9f Info 0x150e6a 533 0 backupd: (TimeMachine) [com.apple.TimeMachine:MountedDestinationManager] Error Error Domain=com.apple.backupd.ErrorDomain Code=29 "(null)" UserInfo={NSUnderlyingError=0xb0000077ec54090 {Error Domain=NSOSStatusErrorDomain Code=80 "EAUTH: Authentication error"}} not retryable.
2026-03-24 18:12:48.617937-0700 0x3f7ea Debug 0x150e6a 533 0 bac
Now, I am really, really, REALLY, REALLY, sure that both the password to connect to the NAS and the password to unlock the (encrypted) volume are correct. I have deleted and recreated the TM connection in the TM control panel approximately 39,241 times and it doesn't help. And this same thing happens on 5 different macs connecting to two different Synology NASs all running the latest DSM.
I can mount the volume in the finder just fine. All of the folders are there. Attempting to browse in TM itself just shows local snapshots. DSM is configured as recommended and has been so for ten thousand years.
group.com.apple.NetworkAuthorization.ServerMarkers (and therefore) serverMarkers.plist do not exist on my Mac…
I'm not aware of what they are supposed to do and how they function (and there isn't much out there if you Google them up).
Did you have to create them yourself or were they already there on your system?
As a side note, nas.localdomain needs to be replaced by whatever is used on your LAN (like MyNAS.local or whatever).
MrHoffman wrote:
PS: The Time Machine NAS configurations of my acquaintance (circa 26.6.x) have not required any adjustments.
Indeed, three of the Macs in my house are on 26.6.2 and all continue their normal, hourly TM backups to my Synology NAS.
Also, the password is simple ASCII characters.
nm i see you resolved it
With what command would I remove a server from the serverMarkers.plist e.g. in case I change the Synology? Thanks!
Thanks, but I am a bit confused now: Isn't that the very command used to add a server to the plist? I'd like to remove one... Thanks!
PS: The Time Machine NAS configurations of my acquaintance (circa 26.6.x) have not required any adjustments.
Excellent, thank you!
MacOS 26.4 has broken TimeMachine backups to NAS.
