Acronis permissions: can't delete acronis files after they have been backed up to another server

My acronis 2017 generates nightly backups using a differential chain scheme. this works perfectly and has saved me after disasters. These backup files are written to my NAS.

My institution backs up select folders the folder on my NAS containing my acronis backup files.

The problem is that I can't delete these backed up files because an unknown account is the owner of the file - not me.

I think this is an Acronis permission issue. I think that Acronis is the owner of the files. Because acronis doesn't have an account on the backup server, I can't delete old backup files and so my share is filling up.

Has anyone come across an issue like this? What are the minimum permissions acronis needs to do its job?

Many thanks