Why does RH-SSO server log a KC-SERVICES0067: failed to parse RestartLoginCookie error?

Issue

We just upgraded from RH-SSO 7.0.0 to RH-SSO 7.2.1 and we are seeing the following error:

2018-04-11 22:51:06,587 WARN  [org.keycloak.keys.DefaultKeyManager] (default task-9) KID is null, can't find secret key
2018-04-11 22:51:06,588 ERROR [org.keycloak.services] (default task-9) KC-SERVICES0067: failed to parse RestartLoginCookie: java.lang.RuntimeException: java.lang.RuntimeException: java.security.InvalidKeyException: key is null
        at org.keycloak.jose.jws.crypto.HMACProvider.verify(HMACProvider.java:87)
        at org.keycloak.protocol.RestartLoginCookie.restartSession(RestartLoginCookie.java:157)
        at org.keycloak.services.resources.SessionCodeChecks.restartAuthenticationSessionFromCookie(SessionCodeChecks.java:359)