Cannot find the user '', because it does not exist or you do not have permission

Run this:

USE [db_where_you_need_access]
GO
CREATE USER [your_user] FOR LOGIN [your_user]

I just encountered the same problem.

Make sure there is an user mapping for your login in the master table. No database role memberships are required, just tick 'Map' for the database master under 'User Mapping' in the properties for the login 'UserName'.