When you use NTNAME in section access you always need to specify users in the form DOMAIN\USERNAME, uppercase. The best way to make sure the user is correct is to create a new text object, and set this function to see what user you have to set in the section access:
Note that the user valid in your computer may not be in the local security group Administrators or QlikView Administrators in the computer where the Server is running. So besides adding yourself as an ADMIN (which makes sense because you have to open and modify the file) you need to add the DOMAIN\USERNAME of the user under the services are running. You can check that in the Start button, Run, Services.msc, right click on any of the QlikView services, Properties, Logon tab.
Once you have added the administrator user that runs QlikView services, reload the file in order to apply changes to section access.
If you are using a reduction field in your section access, note that all users that open the document via AccessPoint or Plugin, or Desktop as client are not ADMIN but USER, even when in the field ACCESS you have set them ADMIN. That means that the administrator user, since he has no values in the reduction field, will not be able to open when Strict Exclusion is checked. One quick solution is to set "*" as value in the reduction field.
Hope that helps.