Time Machine backup fails after copying some files

My Time Machine backups started consistently failing, but only after copying a certain number of files to the backup disk.

Repeated attempts to back up would fail more quickly, almost as if a particular file were causing the failure.

The error messages say to try to repair the backup disk, similar to

Time Machine couldn't complete the backup to "MyBackupDrive". An error occurred while copying files. The problem may be temporary. If the problem persists, use Disk Utility to repair your backup disk.

However, repairing the backup disk failed to resolve the issue. Reparing my boot disk and using a different backup disk also failed.

I am not (to my knowledge) using any services or applications known to cause Time Machine problems, as detailed in answers to related questions.

How can I get the backup to succeed?

The issue may be a corrupted file. Open Console.app and filter errors for "backupd".

You may notice an error related to a particular file which is causing the failure, for example

Error: (-48) SrcErr:NO Copying /Volumes/com.apple.TimeMachine.localsnapshots/Backups.backupdb/mycomputer/2020-04-23-231950/Macintosh HD/Users/myusername/bad_file to /Volumes/MyBackupDrive/Backups.backupdb/mycomputer/2020-04-23-231950.inProgress/A93DB1E9-8D23-482D-953E-0D808B6380AD/Macintosh HD/Users/myusername/bad_file

If so, you can try deleting this file (completely).

In my case, to be safe, I also restarted my machine, reformatted the backup drive (OS X extended, case-sensitive, journaled, encrypted) and re-added it to Time Machine, but I am not sure if any or all of those steps were essential.