Sql Server Error:A connection was successfully established with the server, but then an error occurred during the login proces

To Solve this Error do the following:

1-Open SQL Server Management Studio and login

2-From Server Name right click and Select properties

3-Select Security in left menu and set server authentication to SQL Server and Windows Authentication.

3-Stop SQl service(MSSQLSERVER) and Start it again.
4- Open SQL Server Management Studio. Do not login, cancel the login dialog.
5- From the file Menu select New->Database engine query, and login(windows Authentication).
6-Execute the query ALTER LOGIN sa WITH PASSWORD = ''; to reset the password (if the sa is not enabled then type ALTER LOGIN sa ENABLE to do so).
7-Login with the sa user and add the Administrator user.