Found that some time ago. This error was reported because a run condition given by the 2 instances was trying to delete the same archivelog file. This can vary to more than 2 instances, depending on your RAC configuration, of course.
This is a quick way to identify if you are in presence of the same situation and ignore the error, using the sequence number reported on the error (SEQ) and the last line number reported by grep (LINE):
grep -n SEQ /oracle/greporadb/diag/rdbms/greporadb/greporadb2/trace/alert_greporadb1.log | tail tail -n +(LINE-10) oracle/greporadb/diag/rdbms/greporadb/greporadb2/trace/alert_greporadb1.log| head
grep -n SEQ /oracle/greporadb/diag/rdbms/greporadb/greporadb2/trace/alert_greporadb2.log | tail tail -n +(LINE-10) oracle/greporadb/diag/rdbms/greporadb/greporadb2/trace/alert_greporadb2.log| head
The result should be the archivelog sequence deleted on one of the nodes and the error reported on the other, at the same timestamp.
Hope it helps!