Check the Login's database mapping at the Server Level:

Expand the Server's Security folder (not the Database's)

, then expand the "Logins" folder

, then select and right-click the "SomeUser" login

, Select "Properties" for the popup menu

On the Properties dialog, select the "User Mapping" view

Now check the list for your database. Unless SomeUser is a SysAdmin, it needs to be mapped to the database here.