Backups & restore points · Encryption and passphrases
What does "The passphrase is correct, but Storage Studio couldn't save it securely" mean?
Short answer: Your passphrase was correct, but it couldn't be stored securely on this Mac at that moment, so it wasn't saved. Your backup is unchanged — try Re-enter passphrase… again, restarting the Mac if it repeats.
Your passphrase was right — it unlocked the backup — but the Mac didn't let Storage Studio store it securely on this Mac at that moment, so it wasn't saved. Nothing about your backup has changed.
Click Re-enter passphrase… and try again. If it keeps happening, restart the Mac and retry; a passphrase is only ever saved after it has been confirmed against the backup, so retrying is safe.