Time Machine keeps validating backup, how to identify reason?
I was suspecting a broken file system on the Time Capsule, but needed proof.   After yet another mysteriously cancelling verification step, I found https://superuser.com/q/1126990/7401 which ended up in
log show --last 3d --style syslog  --predicate 'senderImagePath contains[cd] &quot;TimeMachine&quot;' --info|less

which in turn let me find this log line:
2021-04-05 00:48:38.359222+0200  localhost backupd[276]: (TimeMachine) [com.apple.TimeMachine:General] fsck returned an unknown termination status: 8

So, the file system check program reported an error message but Time Machine didn't let me know.  Apparently 8 indicates unrepairable error, which is a rather bad sign.  I then mounted the &quot;Data&quot; drive holding the backupbundle file on Time Capsule manually, and as Disk Utility did not want to mount the backupbundle, I found https://apple.stackexchange.com/a/39842/3157 resulting in
sudo hdiutil attach -nomount -readwrite /Volumes/Data/XXX.backupbundle

giving
/dev/disk2              GUID_partition_scheme           
/dev/disk2s1            EFI                             
/dev/disk2s2            Apple_HFS            

Now the Time Machine backup shows in Disk Utility, and I could run First Aid on the drive which slowly but successfully repaired the filesystem.