ORA-24327: need explicit attach before authenticating a user (DBD ERROR: OCISessionBegin)

One of the OEM agent log has the following error, and some of the jobs fail due to the database connection issues:

$ grep ORA-24327 *
gcagent.log.16:em_error=Could not connect to dbsnmp/(DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=racnode1.virtuallab)(Port=1521))(CONNECT_DATA=(SID=RACTESTDB1))): ORA-24327: need explicit attach before authenticating a user (DBD ERROR: OCISessionBegin)

The output from CA( Corrective Action) job:

Connector descriptor is (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=racnode1.virtuallab)(PORT=1521))(CONNECT_DATA=(SERVICE_NAME=RACTESTDB)(SERVER=DEDICATED)))
Could not connect to SYSTEM/(DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=racnode1.virtuallab)(PORT=1521))(CONNECT_DATA=(SERVICE_NAME=RACTESTDB)(SERVER=DEDICATED))):ORA-24327: need explicit attach before authenticating a user (DBD ERROR: OCISessionBegin)