Mac (macOS Sequoia) cannot create TM backup on Synology DS216j, but macOS Monterrey can

Synology
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Dear All:

I am new on this forum as I have been referred here via a MacRumors thread.

I am using Synology NASes for Apple Time Machine backup destination since many years. Just very recently I ran into the following issue with a Synology DS216j running on the latest DSM 7.3.2-86009 Update 3: I can use it as a Time Machine backup destination on a Macbook with macOS Monterrey 12.7.6, but using the same credentials on an iMac running on macOS Sequioa (macOS 15.7.7) does not work no matter how often I try.

The error I am getting is:

Synology Time Machine issue
Synology Time Machine issue498×582 55.2 KB

“Time Machine couldn’t complete the backup to “TimeMachine”. The network backup disk could not be accessed because there was a problem with the network username or password. You may need to re-select the backup disk and enter the correct username and passwort.”

However, as I said I am using the very same credentials for a non-admin user which only has read/write access to the ‘TimeMachine’ backup folder. I can use these very credentials without problem on both machines to access the respective folder via finder. Just the Time Machine service does not work on the iMac.

  • SMB is enabled

  • the ‘TimeMachine’ folder selected as Time Machine folder in DSM

  • the Time Machine user has read/write permissions on that ‘TimeMachine’ folder

Thanks a lot for any ideas,

Magnus

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Now one of the two drive slots tends to loose its drive after some time, regardless of what drive is in there, so I guess the DS is past its best days…

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So, back to square one: I now set up a Synology DS116 from new and still cannot create TM backups on the iMac. Same error message! As if the iMac is cursed…

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Update: the new Sequoia (macOS 15.7.8) released this week does not solve the issue.

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Further update: a second Mac running the newest Sequoia (macOS 15.7.8) could also not backup to the Synology. So I am pretty sure it’s a Sequoia thing.

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New update, and finally a solution:

Sequoia 15.7.9 does not fix it either, However, the following Terminal command followed by a restart does:

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’

NOTE: Its important to state ‘Add :your-nas-hostname.local bool true’ instead of ‘Add :your-nas-hostname.localdomainbool true’ as I have read also.