Time Machine automatic backup no longer working - error message: "The backup disk image could not be created."

Since upgrading too macOS Big Sur my Time Machine backup process has stopped working across all of my devices.

I've been able to successfully backup manually using an attached USB drive, but the automatic backup over wifi, which I've been using successfully for the past several years, no longer works.

Error message I get is "Time machine couldn't complete the backup. The backup disk image could not be created."

I've taken the following steps to try and resolve this issue myself:

• I've restarted all associated devices (laptops, NAS, etc).
  • I've turned off time machine, restarted, turning time machine back on, etc
• I've created a new backup folder and restarted time machine
• I've tried these steps on two different laptops (both were updated to macOS Big Sur at the same time)

I'll continue to backup manually, but if anyone has an idea of what I could try to get the automatic backup process working again would be greatly appreciated.