I have run into a strange problem twice already. A client’s Altaro off-site backup stopped working and the error was that the off-site server’s FQDN could not be resolved. In both cases OSB was working and resolving the FQDN just fine until it just stopped working one day.
The oddity is that the server that hosts the Altaro off-site backup is able to resolve the FQDN. I even disabled the browser’s DNSSEC setting to make sure that the browser was using the local domain’s DNS resolver, to no avail. Ultimately, I changed the FQDN for an IP address in the OSB settings and the OSB started to back up off-site once again.