hi there, this one did the trick for me.
im currently using:
FSX:SE only
Windows 8.1 64b
PMDG 777-200 base with 777-300 extension.
previously not working with black panels and no landing gears; tried uninstall, reinstall but to no avail.
then i tried to install SimConnect all three of them from the ..\SteamLibrary\SteamApps\common\FSX\SDK\Core Utilities Kit\SimConnect SDK\LegacyInterfaces folder and a couple of restarts and viola! all seems to be working.
hope this one works for you too.