The following are various reasons to get 14420 error.
1) Backup Share permission issues for SQL Server Primary Main Service Account.
2) Network issue on Backup Share.
3) Disk Space issue on Backup Share
4) Service Account change on Primary.
5) Backup Job Owner change
6) Backup job schedule change.
7) Primary Agent Service Account is unable to inform Monitor server about backup task status. (Applicable only if Monitor is configured).
8) Primary database recovery model changed to SIMPLE.
9) Database Corrupt/Suspect on Primary
10)SQL Server Agent is down.