a server must have hung while processing the dml Queue, leaving it locked
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Try the following links for this error:
http://docs.oracle.com/cd/E16655_01/server.121/e26079/drgus.htm
https://community.oracle.com/message/81896
https://community.oracle.com/message/1026483
https://community.oracle.com/message/1924270
https://community.oracle.com/message/1919903
https://community.oracle.com/message/1899030
https://community.oracle.com/message/1051762
https://community.oracle.com/message/130222
https://community.oracle.com/message/1799964
https://community.oracle.com/message/1999917
https://community.oracle.com/message/1999785