Private, invite-only rooms. Messages are encrypted in your browser before they're sent — the server only ever stores/relays ciphertext.
Your session is still valid, but a page reload always starts with no encryption key in memory (it's derived from your password, never stored). Enter it again to unlock — this stays entirely in your browser, it isn't sent to the server.
No encryption key unlocked yet — enter your password above.
Your password derives a different key than the one this account registered with, so its rooms can't be decrypted here. This can only happen for an account whose public key wasn't itself derived from its password (e.g. one created before this scheme, or via a direct API call) — register a fresh account to get a working one.