Minimum user rights for connecting MS Sql database remotedly
Check for "CONNECT" rights

SELECT SUSER_NAME(grantee_principal_id), * 
FROM sys.server_permissions WHERE type = 'COSQ'
GO
USE MYDB
GO
SELECT USER_NAME(grantee_principal_id), * 
FROM sys.database_permissions WHERE type = 'CO'


To fix as needed

USE master
GO
GRANT CONNECT SQL TO myLogin
GO
USE MYDB
GO
GRANT CONNECT TO MyUser
GO