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ORA-03113: EOF on communication channel or "packet write failure"

An ORA-03113 occurs when a session's process dies (or is killed).
This is usually caused by an ORA-00600 or an ORA-07445. Both errors should be resolbed by Oracle support.
ORA-00600 as well as ORA-07445 write a trace file in the background_dump_dest or user_dump_dest.
The 3rd file to look at in such a case is the alert log.