Time Machine/SMB connectivity issue

Hi,

I am pulling significant amounts of hair out attempting to get Time Machine to work on a MacBook - I hope someone can provide a helpful pointer in the right direction, please!

The scenario is that I am trying to configure the MacBook to back up to a QNAP NAS offering an SMB file share. This exact setup is working fine on another MacBook (to the same NAS) - so I don't believe there is anything wrong with the overall network, the way the NAS is configured, or the process I'm going through in trying to set this up.

I am able to add the file share as a backup location, but the initial backup fails with the error below (essentially 'backup disk could not be accessed - problem with username/password'). I know that there is not a problem with the username/password, because:

a) The process for setting up an SMB share as a Time Machine location involves connecting to it, and this works.

b) I can manually connect via finder and create files in that location.

c) The logs on the NAS (screenshot below) show that the Mac connects and even creates and deletes some test files as part of the backup location selection process.

I have tried removing the config on the NAS and recreating it from scratch (verifying it against the config for the working Mac) as well as removing all references to the NAS from the keychain to make sure that there are no out of date credentials anywhere, with no effect. I also can't find any logs that show any errors.

I'd be very grateful for any tips about what to try next, please?

I am a Linux sysadmin by profession. I don't know MacOS that well, but I'm willing and able to dig if I knew where I was looking.

Thanks,

Andy