Thanks for your suggestions. I omitted to mention, that Erpnext runs in Docker.
Hence checking through the image, I could not find any settings, which put the right download address in the email message “Please use following links to download file backup”.
The links below is typically the docker location. Where can I set the proper address?
Public Files Backup: http://site1.local:8000/backups/20190813_210816-site1_local-files.tar
Private Files Backup: http://site1.local:8000/backups/20190813_210816-site1_local-private-files.tar
Meanwhile I did found a reference to “site1.local” under “/frappe-bench/sites/currentsite.txt” which containers “site1.local”. However unclear is yet where this address is set?
Thanks for your help