Time Machine couldn't complete the backup. Time Machine could not delete the backup disk image.

I am backing up my wife's MacBook air (Monterey 12.6.3) and my MacBook Pro (Ventura 13.6.6) to a WD EX2Ultra NAS. Has worked fine for several years. My backups are working fine. The WD NAS disks are not full.

Recently the MacBook Air backup stopped working. First message seemed to indicate the backup image was corrupted. The message when I try to run the backup for the MacBook Air is:

Any suggested actions I can take to get the MacBook Air backups working again are much appreciated.