I have never tried running FDC in FSX until today - and I find it won't connect, with the same error message as in this (old!) thread. I have downloaded and installed the latest FSUIPC files, though they were all fairly recent already) and have looked at the 'registry fix' suggested on the FAQ page (but my value is already set to 0x23c00 value).
I get the same error when I try to connect PF3 to FSX as well, but I won't post a separate thread there unless you want me to, Dave...
I assume there is more I can do? I've browsed a number of threads here - if I missed 'THE' solution, apologies,
Martin