Time Machine backup fails after copying some files
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.