Did you just change your authentication to DMS? If so, it's important that you specify AD user names in the Authorization tab in the correct format, which for some reason can be very finicky. For example, if the PC is a member of the testdomain.com domain, and the user is Joe, you can specify this user as either Joe or testdomain\Joe, but not testdomain.com\Joe.
Otherwise, if NTFS permissions and DMS settings are correct, a restart of your services should fix the problem.
Does it give the error for all users or just for some? If this is not your Domain Controller, maybe check the trust relationship between the servers. It's also possible that the DSC failed for some reason (i.e. QVS locked itself out of the documents). To see the directories for the AccessPoint files: QEMC --> System --> QlikView Servers --> Folders tab. Check NTFS permissions on the files. Specifically, make sure that the Publisher account has full access and that the proper users have at least Read/Execute access.
It's still not clear to me if you are using DMS authentication...? Try stopping all QV services, deleting the META files for your documents (located in the same directory as the files with a .META extension), and restarting the services. When you open the AP, QVS should automatically recreate clean META files. It's possible that they got corrupted somehow.