Time Machine fails shortly after starting to prepare backup
For others who may read, the following will give you 24 hours of Time Machine logs...
log show --style syslog --last 24h --predicate 'senderImagePath contains[cd] &quot;TimeMachine&quot;' --info

I stumbled across your post because I've gotten the same thing, which is not the first time.  Apple has broken Time Machine in previous OS updates as well.  I'll bet this started when you upgraded to 11.3, as I did.
I see other errors which probably give more info...
localhost backupd[1175]: (TimeMachine) [com.apple.TimeMachine:EventCollection] Event store UUIDs don't match for volume: Macintosh HD - Data ...

and also...
localhost backupd[1175]: (TimeMachine) [com.apple.TimeMachine:EventCollection] Failed to enumerate snapshot diffs, error: Error Domain=NSPOSIXErrorDomain Code=22 &quot;Invalid argument&quot;

localhost backupd[1175]: (TimeMachine) [com.apple.TimeMachine:EventCollection] Failed to collect events for &quot;Macintosh HD - Data&quot; (device: /dev/disk1s2 mount: '/System/Volumes/Data' fsUUID: ... : ...), error Error Domain=com.apple.backupd.ErrorDomain Code=304 &quot;(null)&quot; UserInfo={FailedStrategy=32}

localhost backupd[1175]: (TimeMachine) [com.apple.TimeMachine:General] Backup failed (304: BACKUP_FAILED_PREFLIGHT_STAGE_SNAPSHOT_DIFFING)

I would guess our friends at Apple are learning the hard way as others have done that UUIDs are a bad means of storing a unique ID, because they wind up changing for unknown reasons and then your thing is broken.
In any case, the last in the string of failures for me was an invalid excluded directory...
localhost com.apple.prefs.backup.remoteservice[37280]: (TimeMachine) [com.apple.TimeMachine:General] Failed to size '/Users/robertclayton/VirtualBox VMs', error: -43

I would classify this as a bug as well, since a directory being excluded doesn't need to be &quot;sized,&quot; after all I told it to exclude the folder there's no reason for the OS to want to know how big it is.
So if you've restored a backup recently, check and see if you have excluded folders in your Time Machine settings that are no longer present. If you do, on 11.3, apparently that causes the backup process to fail. After I removed the non-existent excluded folder mine started back up again normally.