I've found that having Simple File Sharing enabled on certain computers causes a WMI Access is Denied error when scanning. To solve this issue, you can turn off Simple File Sharing via the Control Panel > Folder Options > View tab.
OR
You can create the following GPO to automatically turn off Simple File Sharing:
Computer Configuration > Policies > Windows Settings > Security Settings > Local Policies/Security Options > Network Access
Change "Network Access: Sharing and security model for local accounts" to "Classic - local users autenticate as themselves"
Hope this helps some people having problems...