How to create a login and user that can only access one schema in one database
Off guess you are missing the connect permission.  I'd always thought that was created with the user but there may be conditions where it isn't added.

Try running

USE [MyDatabase];
GO
GRANT CONNECT TO [MyDomain\MyAccount];


You could also try changing the default schema of [MyDomain\MyAccount]

ALTER USER [MyDomain\MyAccount] WITH DEFAULT_SCHEMA = MySchema;


Lastly you can use my sp_dbpermissions and sp_srvpermissions to review the permissions for the user.

EXEC sp_srvpermissions 'MyDomain\MyAccount'; -- Return instance level permissions
EXEC sp_dbpermissions 'All','MyDomain\MyAccount'; -- Return DB level permissions for all databases