As pennance for sending an email to Pete instead of posting my question in the forum first (I did it only because I figured he would want to eventually see my zipped copy of my fsuipc4.key file, as he requested another poster to send him), I will post my "discovery" after over 8 hours of angst at getting the dreaded FSC "Unable to connect to Flight Simulator Msg# (no process started)" error message in FSC v8.6. I don't know HOW it happened, or WHY it happened, but it DID happen...even after following the instructions in the FSCommander forum several times with no success at solving the problem.
I downloaded and installed the FSC "full version" 8.6 installer. Yes...I did "Run as Administrator" when I ran it. It seemed to install fine. It even put the 2 shortcut icons on my desktop for me (for FSC86.exe and FSCDbManager86.exe). I then copied the 2 files that I had backed up (per the instructions) from my previous FSC installation into the new main FSC folder . Then I ran the DataBase Manager, per the instructions. All seemed fine. But FSC would never connect to FSX.
After over 8 hours of anguish including checking and double-checking all my settings for "Adminstrator" Rights, etc, I stumbled across something. When I right-clicked on the 2 desktop shortcut icons the installer placed there and selected "Properties", the "Target" and "Start in:" boxes were BOTH BLANK. I have no idea how or why that happened. Or why when I clicked on them, they would BOTH run the programs yet. But they did. And the FSC86.exe one would load a 021209 version, which I discovered was not the correct build.
So I deleted the 2 desktop shortcuts the installer put there, went to the FSC folder where the EXE files were, and dragged them to the desktop myself and did a "Create Shortcut Here" for them. After that, EVERYTHING worked fine. And the Build for FSC was then shown as 101209...the correct one.
If you are still having "connection" problems with FSC v8.6, check your desktop shortcut properties for the 2 shortcuts. You may need to delete them, create new ones directly from the FSC installation folder, then double-check the security settings for the two files. But it appears now, at least in my case, it was not a problem with FSUIPC.
If this helps anybody, you're welcome! :)