You execute a Backup from within a Backup in your PostExec.
Better don't do that and schedule them separatly.
Until the Session is completed some Locks may perisist and cause trouble.
Can you set up a small BackupTest? one Drive, no Mirror, some Files/Folder on your MediaServer.
Make sure Drive is clean and won't be used by backups soon and run the Test twice after each other. Let me know if it still faults.