Create a new Contact API causes http500 error

Document ID : KB000092837
Last Modified Date : 16/05/2018
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Creating new contact from webservices_rest causes following http 500 error

message: Internal Server Error
description: The server encountered an internal error (Internal Server Error) that prevented it from fulfilling this request. 

This is caused because there is same login_name is on CM_CONTACT table. 
However, this user is not shown on UMP=>Control Panel=>user and organization. 
This is caused from Index is not found for this user. 
Please create Index for Liferay tables by control panel > server administration > reindex all search indexes.
Now the hidden user should be available on control panel. Please delete this user from user and organization. 
After deleting the user, there should be no problem on creating user from webservices_rest.