Time Machine no back up for 22 days, updated 25 days ago to 6.10.7
Time Machine on Monterey (and probably earlier) might have an issue with notifications. Just had the first notification about a Time Machine error backing up to a ReadyNAS 424 - it hadn't backed up for 22 days. There has been no obvious notification until today.
Time Machine might give a notification but the other clue is a red info box that appears in the System Preference panel. But that's it. So the warning is double check the last date of back up.
However, this seems to coincide with the last Netgear update 6.10.7 which was 25 days ago. Had not had this issue with the prior release.
The exact error was that it was having problems accessing the sparse bundle (error 16). A reboot of the 424 solved the problem. I'm watching backup progress now, so it seems to be working.
Unclear whether 6.10.7 introduced an issue with Time Machine access to the backup image.
Just check the dates of your last back up since the failure can go unnoticed.