I recently had this error (while troubleshooting account credentials) in the CM12 console when running “Test Connection”. I was attempting to validate the account credentials using C$ of the site server, but it generated error 0x800704c3 for “Multiple connections to a server or shared resource by the same user, using more than one user name, are not allowed”.
The resolution? Don’t use a hidden share! Instead, I tested the connection with the default SMS_sitecode share and it worked.