The following are various reasons to get 14421 error in log shipping.
1) Backup Share Permission for SQL Server Standby Agent Account.
2) Network issue on Backup Share.
3) Disk space issue on Local Copy Folder.
4) Service Account change of Standby
5) Copy/Restore job owner changed.
6) Copy/Restore job schedule changed/disabled.
7) Copy/Restore jobs are unable to inform Monitor server about the status. (Applicable only if Monitor is configured)
8) Manual Log Backups; LSN Chain Missing ; Corrupt Log Backups
(Restore job gets affected)
9) Restoring Log Backup WITH RECOVERY by mistake.
10)TUF file corrupt restore job will fail.
11)SQL Server Agent down.
12)Disk space issues on the Standby database location.
13)Standby database corrupt.
14)Standby is being continuously used for Offload Reporting tasks.
15) When restoring on Standby a file was added on Primary in a different path. When restoring on standby mention WITH MOVE.