"Restart login cookie not found" in Keycloak
"Restart login cookie not found. It may have expired; it may have been deleted or cookies are disabled in your browser." Keycloak shows this on a dead-end HTTP 400 page, and the message is more confident than it should be. It means exactly one thing: the request arrived with no KC_RESTART cookie at all. Not a stale one, not a rejected one. None. That cookie has no expiry to run out, so "it may have expired" is the one explanation that is never true. A rejected cookie produces a different message. And the page tells the user to "Click Back to Application", which on a default client renders no link to click.
Here is what the cookie is for, how to tell in one log line which failure you have, and the one client field that turns the dead end into a recoverable page.