Time Machine failing no error
It sounds like you may need to dive into the logs to get some more info.

MacOS uses unified logging so the logs aren't stored in a specific file. In Terminal, this command should print all TimeMachine backup failure messages:

log show --predicate 'subsystem == "com.apple.TimeMachine"' --info | grep 'Backup failed'


However this cuts out quite a bit of information, so you may need to use the broader search:

log show --predicate 'subsystem == "com.apple.TimeMachine"' --info | grep 'Error'


Running this on my machine showed TimeMachine to be quite noisy so it may take some time to find something useful in the output especially if you have been running TimeMachine for a while

Post back here with any errors you find.