RMAN-06169, but only when backing up via OEM 13c

backuporacleoracle-12coracle-enterprise-managerrman

When running a Level 1 or Level 0 backup via OEM 13c, the following RMAN error causes the run to fail. (The RMAN script is being ran with proper sysdba credentials from OEM 13c.)

RMAN-06169: could not read file header for datafile 2 error reason 5

Oracle documentation suggests reason 5 indicates 'Unable to open file'.

However, if I execute the exact same RMAN statement directly on the server, the backup runs fine.

We have removed the database from OEM and decommissioned the agent, the redeployed the agent as well as reinstated the database in OEM, but the error still occurred.