Backup: Error backup job Documenten: Failure during copy.

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Since a week my backup job failed every night :

The backup-log said:

Backup Status: Fail: Failure during copy.

Cannot copy Financial/overview/yearpverview_2020.pdf! Failed sending data to the peer

Cannot copy Financial/Archive/dataview.xls! Transferred a partial file

Most of the files 9>1.000) are copied succesfully, but approximately 11 files are not copied because of errors: "Failed sending data to the peer" or "Failed sending data to the peer".

I can read and access these files from the RN104 in a normal way. eg. with Acrobat-reader or with MS/Excel.

The backup job is a of type FTP and it send every night data to the same USB-disk which is plugged in, into a router.

Does anyone know what the errors mean?

Is the NAS corrupt or is the USB-disk corrupt?

Or is somthing else wrong.

What can I do to fix this problem.

I use a RN104 with latest OS: 6.10.5