I had a similar problem with Big Sur and my Synology NAS. Time Machine kept failing with various errors. After much investigation it turned out that the combination of NAS, WiFI, Big Sur and Norton was provoking the issue. I ascertained that if I disabled Norton, Time Machine would run correctly. It was strange as there was no problem with the same version of Norton beforehand. It was very annoying, making my new NAS useless. I spent hours on the phone with Norton as they run various tests but simply said the problem would be passed to engineering but nothing happened. So I removed Norton and installed the trial version of F-Secure. All has been working great for 6 months.
Robert
