This error is thrown when the server has been terminated, was forced to terminate or the server timed out the connection. You should check if the server session was terminated and whether timeout parameters are set properly in sqlnet.ora. Check if you have firewall issues or whether something cleans TCP idle connections.
Check the timeout parameters in sqlnet.ora and especially sqlnet.expire_time, which value you could set to 1 or 10. Modify
SQLNET.INBOUND_CONNECT_TIMEOUT = 300
SQLNET.SEND_TIMEOUT = 300
SQLNET.RECV_TIMEOUT = 300
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