You can check about the orphaned user if you look into the dnn database --> Security --> Users --> right click on the users Name and select advanced Settings.  Then check if the user have mapped to a Login Name.  If not run this as an  query on SQL Management Studio:

use <databaseName>

GO

EXEC EXEC sp_change_users_login 'Update_One', '<UserName>', '<LoginName>';

GO

You have to Change the values into < > to yours!

I checked the users and none of them are orphaned users. Everything seems to be fine there. I've tried everything. Can't see why this isn't working! Very frustrating.