Cause
You are tyring to login while another user is mounting or dismounting the same database.
You are tyring to login while another user is mounting or dismounting the same database.
Check to see if the database is in the middle of being mounted or opened and try your login again once the database is accessible.
Try the following links for this error:
http://docs.oracle.com/cd/E16655_01/server.121/e26079/e12300.htm
https://community.oracle.com/message/1222965
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