failed with code208

I meet a recurring problem during my backups.
I created a working incremental backup that run every day.
But I encounter an error message each automatic launch: “Backup” E: \ backup_inc_files.cbu “failed with code208”
I do not have this problem when I run the job manually.
I save two NAS servers on an external USB hard drive. To backup my servers, I mounted network drives on a Windows 10 computer on which is installed Comodo Backup. The backup disk is connected to the same PC.