Oracle Error Message
ORA-16075: standby database destination mismatch
Reason for the Error
Another instance had access to a standby database destination that this instance did not. If the initialization parameter LOG_ARCHIVE_CONFIG does not define a DG_CONFIG, this may be due to a standby database becoming unavailable recently.
Solution
Ensure that at each instance the set of standby databases referred to by LOG_ARCHIVE_DEST_n initialization parameters is the same and define a DG_CONFIG at each instance to identify those databases explicitly.