Archival Error Received logging on to the standby

I got email from our OEM such as ” PROD EM Event: Critical:XXXDG – The database status is UNKNOWN.” This standby db is belong to of our 11.2.0.4 RAC database which is running on AIX 7.1. While I checked node 1&2 alert log, there are some message such as: ARC0: Archive log rejected (thread 1 Continue reading