A backup failed or a restore was refused

What you see. “The last backup failed: …”, or Restore refuses a file.

Why and what to do.

“The last backup failed: …”. The backup folder can’t be written, for example a cloud drive that is unavailable. Fix access, or choose another folder with Settings → Backups → Change….
“This file isn’t a Tented journal backup.” You picked the wrong file. Pick a tented-journal-….sqlite file.
“This backup was made by a newer version of Tented…”. Update Tented first (see Update Tented).
Older copies disappear. By design: automatic backups keep the last 14 days plus one a month for a year. Each computer removes only its own copies.