Cannot open Unencrypted Time Machine Sparsebundle on MacBook Air Running Sequoia 15.4.1

First, make a new admin user, log out and into the new account, does it work there?

Have you tried disabling the firewall?

jgaeddert wrote:

Background: I am trying to get Time Machine set up to back up my M1 MacBook Air “Proteus” running 15.4.1 (24E263) on a network-attached storage (NAS) system. It always fails the initial backup giving a cryptic “The backup disk image ‘Proteus.sparsebundle’ could not be accessed (error 49180)”. My Intel iMac has been backing up to the NAS successfully for months. Additionally my wife's identical M1 MacBook Air successfully backs up to the NAS.

So I started poking around

Stop right there. I've seen this more times than I can count. You're deep in a rabbit hole and asking for a better shovel. The only way out is up.

My question is this: how do I go about identifying what component of macOS Sequoia is corrupt? and how do I go about fixing this? I’m not keen on reinstalling the entire OS. 

You see what I mean?

Go back to the start. The problem is the NAS. You will need to create a new mount point for your backups. Permissions are always complicated and you're doing it across two machines with two different operating systems. That's never going to work. However you setup the first Time Machine backup on the NAS, do all that again for your user. I know nothing about your NAS to tell you how to do it. If it were a standard Linux server, you would just create a new Linux user, create a new mount point owned by the user for your backups, then access that user and user's password from your Mac account.

Ah, yes, hmmmmm…

Have you tried inherit the things ?

https://tommyang.github.io/pondini.org/TM/B5.html

https://tommyang.github.io/pondini.org/TM/6.html

Ah.