The problem is that the Master Page Gallery for the site collection needs to have the permissions changed. You can verify that this is the problem by
- Navigate to your Master Page Gallery (http://yourSPhostHere/_catalogs/masterpage/Forms/AllItems.aspx).
- Once there you can get permissions on that library. Next Click the Check Permissions button and enter the user that is being denied access. If you see that they don't have access then this is likely your problem.
The solution is simple. From the permissions for the Master Page Gallery do the following:
- Click the Grant Permissions button.
- For Users/Groups textfield enter: All Authenticated Users
- For Grant Permissions select Add users to a SharePoint group (recommended)
- Select Style Resource Readers from the drop down list.
- Click the OK button.
- Try to login again. This time it should work.