Email support@ballastbackup.app. It helps to include the command and its output: in the app, open the job and tap the copy button next to the command, or Settings › Command log › Copy log.
With a key, the server needs this phone's public key. When you add a server, Ballast shows a one-line command that adds it; run it on the server as the user Ballast signs in as. With a password, check the username and that the server allows password sign-in.
The server's host key isn't the one saved the first time. That's expected after reinstalling the server. Compare the new fingerprint with ssh-keygen -lf /etc/ssh/ssh_host_ed25519_key.pub on the server, then tap Trust the new key. If you didn't change anything, don't.
The SSH user can read the server but not that backup. Give it read access to the repository or log, or use a user that has it. Ballast never uses sudo.
Each job turns stale when its last snapshot is older than its stale time. If the backup simply runs less often, open the job and use the stale time Ballast observed, or pick another.
Ballast has no server, so it checks when you open it, when you pull to refresh, and when iOS gives it background time, a few times a day. Check that notifications are on for Ballast in iOS Settings; Ballast's Settings will say if they're off. For a check at a fixed time, add a Shortcuts automation that runs "Refresh Ballast".
The trial and the unlock are tied to your Apple Account and shared with your family through Family Sharing. On a new device, open Settings › Ballast and tap Restore purchase.
Only read-only commands that list snapshots and read logs, such as restic snapshots --json, borg list, kopia snapshot list --json and zfs list -t snapshot. Every one is shown in the app with its output.