Does this give a clue refer MSDN Q183366
For an initial connection, if you use NULL for lpPassword or lpUsername, the interactive user's credentials are used by default. If there has been a successful initial connection to the server, subsequent attempts to open a connection using a different set of credentials results in error 1219 (ERROR_SESSION_CREDENTIAL_CONFLICT), which indicates a conflict between the supplied set of credentials and the existing one. To establish a connection using a different set of credentials, first you have to cancel the existing connections to the server using WNetCancelConnection2().