Error 4209 during Incremental Backup

Issue: 3 incremental backups were created successfully and then 4th incremental backups failed with Error 4209. After that full backup was created. So the log looks like as below.

File Full Backup - Success
File Incremental Backup - Failed
File Incremental Backup - Success
File Incremental Backup - Success
File Incremental Backup - Success
File Full Backup - Success

Backup Scheme: After Every 6 incremental backups, perform one full backup.

Error:
Operation: File Incremental Backup

Execution result: Failed

      Error code: 4209

     Description: Abnormal exit during backup/synchronization.

Thank you for your help.