I have the same problem. Both on a Mini M1 and a MacBook 14 Pro M1 Pro.
Backups worked perfectly on Ventura BEFORE upgrade and instal of Sonoma 14.1.
Both make Time Machine backups each hour. To local external disk, and to a networked share.
Backups are 2TB (max size) for the M1 (HDD and network) and 4TB (max size) for the Macbook 14 Pro (SSD and network). In total a maximum of 12TB. All are encrypted.
None of the backup targets are full., but they contain recent history (document versions, code versions etc), that I do not want to loose.
In energy saving, I have specifically set "Don't go to sleep, when screen is closed", and 5 seconds after screen closes, password or fingerprint is required. The M1 has an Apple keyboard with fingerprint reader. Works.
There have NEVER been problems with this setup BEFORE Sonoma 14.1
AFTER, I periodically get the error message: "The file “BackupDisk” couldn’t be opened because you don’t have permission to view it." I have some choice words for that statement.
I can reproduce the following:
If I work on the machines: No problems. TimeMachine will do, what it has to do. So accessing the local drives or remote networked backups is no problem. Credentials work. Data is correct. Time Machine can access ALL four backup targets without problem. I have permission. Unrestricted and any time, the computer screen does not go to sleep.
Whether the backup takes minutes or hours. NO DIFFERENCE.
If I decide, that I do NOT want to sit by the computer for hours on end, just to make TimeMachine Backups possible, and leave the machine to continue on it's own, the screen will of course go to sleep (no pause screen). The display (internal or external) turns off after the specified timeout of 30 minutes (on external power). Works.
I have NOT tested in battery mode, YET, but assume that if you use shorter display timeout in battery mode, you'll experience the problem far, far more often.
The problem seems to exclusively being triggered, byt the display turning off (probably with a five second grace period). When I activate the keyboard, and log in (fingerprint), the TimeMachine backup has stopped, with a red exclamation mark, and the above error.
There are NO credential problems, before the screen goes to sleep (which is standard) and requires a password for login (which is standard basic security procedure). After 30 minutes inactivity and 5 seconds grace time.
Now if I login again, and restart the backup, there are NO problems. And as long as I produce periodic keyboard input - or other activity - on the mashine performing the backup, NO PROBLEMS EXIST, and the backup completes.
As I see it, there's a bug in TimeMachine or Sonoma 14.1's handling credentials for access to encrypted backups (on network or local media), while the displays is off, and the requirement for renewed user login is activated after a five seconds grace period (in my case).
TimeMachine has ALL the credentials it has always needed. TimeMachine can access all backups (remote or local) reliably and consistently, as long as the display is on.
A clear bug. One hundred percent reproducible.
After login, just force Time Machine to backup immediately. It will start the backup without problem. I have all the credentials required. For as long as it is needed, as long as my display does NOT turn off dure to inactivity.
The problem could probably be solved quickly, if Apple released a script, that could be run in terminal, and convinced Sonoma, that the credentials used by TimeMachine are completely kosher.
Or immediately release a security update removing the problem.
In my case, the problem can be reproduced.
In my case the problem does not exist, while doing backups, NOT lasting longer than display inactivity timeout.
In my case the problem does not exist, if the inactivity timeout is not triggered (and the need for password entry/fingerprint turns up).
Please check, if the "activity solution" also solves your problem, BEFORE tampering with backups, that may be perfectly good.
Sigh...
