Thanks for the quick reply Jim. This would be the file in the same location as the scenery.cfg, so ProgramData\Lockheed Martin\Prepar3D v4. I checked the encoding on the saved (and supposedly working) copy, and it's in UTF-8. However place that file into ProgramData\...etc. P3D boots to the black screen. If you delete that and let P3D load it generates a new dll.xml file and it loads just fine. So either I have to uninstall/reinstall the PMDG 737, 777, and 747 plus the Carenado aircraft to get the dll entries back or I have to hand type the entries in. If I copy and past the entries in Notepad++ it black screen boots again.
What's really frustrating is I have no idea what is causing this to happen. Nothing is being changed between uses; no addons being installed or removed-it just stops working.