macOS Tahoe 26.4 introduces several new features but breaks Time Machine backups to network‑attached storage, causing repeated credential errors. A temporary workaround exists, and Apple is testing a fix in the upcoming 26.5 beta.
macOS Tahoe 26.4 introduces new features, but many users now report that it breaks one of the most important tools on Mac, Time Machine backups over network drives, which fail with repeated credential errors even when login details are correct and the storage remains accessible.
Many Mac users rely on Time Machine with NAS devices because it removes the need to plug in an external drive every time they want to back up their data, and setups with brands like Synology, Ugreen, and TerraMaster usually work without issues across multiple Macs and networks.
Now, after updating to macOS Tahoe 26.4, those same setups fail to initialize backups, and the system throws credential errors even though users can still mount and access their NAS normally, which confirms the problem is not related to incorrect usernames or passwords.
Reports suggest the bug first appeared during the macOS 26.4 beta cycle, yet it made its way into the final release without a fix, leaving many users unable to continue their regular backup routines.
Apple Support Community user Nicholas Sayer described the issue as follows:
“MacOS 26.4 has broken TimeMachine backups to NAS. I’ve spent a half hour 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.”
Workaround exists but it is not simple
Some users found a workaround that involves manually editing a system plist file and adding the NAS hostname to restore authorization behavior, which can fix errors like NAConnectToServerSync failed with error: 80.
However, this method requires Terminal access with root permissions, and most users avoid it because it involves modifying system‑level files that can break things if done incorrectly.
A fix is already in testing
There is some relief, as early reports confirm that Apple has fixed this issue in macOS Tahoe 26.5 developer beta, where Time Machine backups over network drives work again without manual changes.
For now, users who depend on NAS backups either need to switch to a local external drive, apply the advanced workaround, or wait for the public release of macOS 26.5 expected later this month.
