@WaffleUncle So you are using the Import button in the app section, correct - https://docs.cloudron.io/backups/#import-app-backup ?
In your case, it looks like your backups were in the file system and not external. If that is the case, you have to fill up the 'Import Backup' dialog manually:
Upload the backup app_client.xxx.tar.gz to somewhere in the server like /tmp/app_client.xxx.tar.gz.
In the import UI, Choose Filesystem
Put the path as /tmp/app_client.xxx.tar.gz
It should import now.