Logs:
2021-09-10 17:28:06  Not starting scheduled Time Machine backup: Automatic backups disabled
2021-09-10 17:28:07  Starting age based thinning of Time Machine local snapshots on disk '/System/Volumes/Data'
2021-09-10 17:45:15  Starting space-based purging of local snapshots on disk '/' - current free space: 896.51 GB (896,505,634,816 bytes), target free space: 1 PB (1,000,896,505,634,815 bytes), initial free space: 896.51 GB (896,505,634,816 bytes), urgency: 4
2021-09-10 17:45:15  Starting age based thinning of Time Machine local snapshots on disk '/System/Volumes/Data'
2021-09-10 17:45:43  Space-based purging deleted Time Machine local snapshot 'com.apple.TimeMachine.2021-09-09-213702.local' on disk '/System/Volumes/Data in 28.0 seconds' - current free space: 898.54 GB (898,540,339,200 bytes), target free space: 1 PB (1,000,896,505,634,815 bytes), initial free space: 896.51 GB (896,505,634,816 bytes)
2021-09-10 17:45:43  Completed space-based purging of snapshots on disk '/' in 28.1 seconds - current free space: 898.54 GB (898,540,339,200 bytes), target free space: 1 PB (1,000,896,505,634,815 bytes), initial free space: 896.51 GB (896,505,634,816 bytes), urgency: 4, remaining snapshots: (
2021-09-10 17:47:14  Initial thermal pressure level 0
2021-09-10 17:51:40  Mountpoint '/Volumes/My Book for Mac' is still valid
2021-09-10 17:51:55  Initial thermal pressure level 0
2021-09-10 17:51:55  Not starting scheduled Time Machine backup: Automatic backups disabled
2021-09-10 17:51:55  TMPowerState: 2
2021-09-10 17:52:30  '/System/Volumes/VM' not offered as a backup destination because it is not browsable
2021-09-10 17:52:30  '/System/Volumes/Preboot' not offered as a backup destination because it is not browsable
2021-09-10 17:52:30  '/' not offered as a backup destination because it is the root volume
2021-09-10 17:52:30  '/System/Volumes/Update' not offered as a backup destination because it is not browsable
2021-09-10 17:52:30  '/System/Volumes/Data/home' not offered as a Time Machine destination because it does not allow read, write, or append access.
2021-09-10 17:52:30  '/System/Volumes/Data' not offered as a backup destination because it is not browsable
2021-09-10 17:52:30  '/dev' not offered as a backup destination because it is not browsable
2021-09-10 17:52:56  Starting manual backup
2021-09-10 17:52:56  Mountpoint '/Volumes/My Book for Mac' is still valid
2021-09-10 17:52:56  Mountpoint '/Volumes/My Book for Mac' is still valid
2021-09-10 17:52:56  Checking identity of target volume '/Volumes/My Book for Mac'
2021-09-10 17:52:56  Mountpoint '/Volumes/My Book for Mac' is still valid
2021-09-10 17:52:56  Mountpoint '/Volumes/My Book for Mac' is still valid
2021-09-10 17:52:56  Backing up to My Book for Mac (/dev/disk3s2,e): /Volumes/My Book for Mac
2021-09-10 17:52:56  Found matching machine store '/Volumes/My Book for Mac/Backups.backupdb/XXXXXXXX (79)' for computer named 'XXXXXXX iMac (79)', no machine store inheritance needed
2021-09-10 17:54:16  Mountpoint '/Volumes/My Book for Mac' is still valid
2021-09-10 17:54:47  Failed to prepare spotlight indexing for '/System/Volumes/Data' (elapsed time: 30.03 seconds)
2021-09-10 17:54:49  Created Time Machine local snapshot with name 'com.apple.TimeMachine.2021-09-10-175447.local' on disk '/System/Volumes/Data'
2021-09-10 17:54:49  Declared stable snapshot: com.apple.TimeMachine.2021-09-10-175447.local
2021-09-10 17:54:49  Mounted stable snapshot: com.apple.TimeMachine.2021-09-10-175447.local at path: /Volumes/com.apple.TimeMachine.localsnapshots/Backups.backupdb/XXXXXXX iMac (79)/2021-09-10-175447/Macintosh HD - Data source: Macintosh HD - Data
2021-09-10 17:55:00  Found existing volume store '/Volumes/My Book for Mac/Backups.backupdb/XXXXXXX iMac (79)/2021-09-06-174437/Macintosh HD' for disk '/System/Volumes/Data'
2021-09-10 17:55:00  Possible strategies for "Macintosh HD - Data" (device: /dev/disk2s1 mount: '/System/Volumes/Data' fsUUID: 87EB482A-B6F5-3F9A-9C2A-FD1E652BE9C9 eventDBUUID: E59C3A04-F682-4779-BB78-0AD01E37922B) (<EventMarker 2021-09-06 20:59:35 +0000 75743201 E59C3A04-F682-4779-BB78-0AD01E37922B 0>): first backup, deep scan, FSEvents,
2021-09-10 17:55:00  Using FSEvents for source: "Macintosh HD - Data" (device: /dev/disk2s1 mount: '/System/Volumes/Data' fsUUID: 87EB482A-B6F5-3F9A-9C2A-FD1E652BE9C9 eventDBUUID: E59C3A04-F682-4779-BB78-0AD01E37922B)
2021-09-10 17:55:00  Read 0 clone families from /Volumes/My Book for Mac/Backups.backupdb/XXXXXXX iMac (79)/2021-09-06-174437/.87EB482A-B6F5-3F9A-9C2A-FD1E652BE9C9.clonedb
2021-09-10 17:55:00  Buffering events...
2021-09-10 17:55:00  Unrecoverable zero fileid - path: 'Users/XXXXXXX/Library/Containers/com.apple.Safari/Data/Library/Caches/com.apple.Safari/WebKitCache/Version 16/Blobs/97829D2F9B149F4B0DBD93B1D000E22F51831C81', flags: 00100200
2021-09-10 17:55:00  Event collection stopped because of error: Error Domain=com.apple.backupd.ErrorDomain Code=303 "(null)"
2021-09-10 17:55:00  Failed to collect events for "Macintosh HD - Data" (device: /dev/disk2s1 mount: '/System/Volumes/Data' fsUUID: 87EB482A-B6F5-3F9A-9C2A-FD1E652BE9C9 eventDBUUID: E59C3A04-F682-4779-BB78-0AD01E37922B), error Error Domain=com.apple.backupd.ErrorDomain Code=303 "(null)" UserInfo={FailedStrategy=16}
2021-09-10 17:55:03  Unmounted local snapshot: com.apple.TimeMachine.2021-09-10-175447.local at path: /Volumes/com.apple.TimeMachine.localsnapshots/Backups.backupdb/Michaelâ  s iMac (79)/2021-09-10-175447/Macintosh HD - Data source: Macintosh HD - Data
2021-09-10 17:55:05  Mountpoint '/Volumes/My Book for Mac' is still valid
2021-09-10 17:55:05  Mountpoint '/System/Volumes/Data' is still valid
2021-09-10 17:55:05  Backup failed (303: BACKUP_FAILED_PREFLIGHT_STAGE_GATHER_EVENTS)
2021-09-10 17:55:27  Backup cancel was requested.