Same problem here.
I made this https://support.plesk.com/hc/en-us/articles/214027529-Unable-to-create-a-backup-on-FTP-Unable-to-upload-backup-files-to-FTP-storage-get-broken-pipe some days ago and worked once, only once, but now doesn't work even if I repeat the steps.
I have "Versión 17.0.17...