GSX fully updated.
Crew and pax are invisible inside GSX Jetways in ALL my 3rd party airports.
In Asobo's airports (i.e. EDDF) they appear but fall from the sky before entering into the Jetway.
Quite annoying. 
PS: 1/ Reinstall according to this thread http://www.fsdreamteam.com/forum/index.php/topic,26933.0.html didn't help.
2/ The correct AFCAD/BGL is being loaded by GSX.
Just to be sure:
1) You have the lastest files from the offline installer?
2) You have excluded GSX jetways from all your 3rd party airports? YES
Also you need to exclude deluxe/premium airports as EDDF, because they are also encrypted. DONE, THANKS FOR THE INFO
3) The 3rd party AFCAD is loaded by GSX (not the default one)? [I know you said it, but to be sure what you consider "correct"] YES, BY GSX, NO DOUBTS
4) You see pax on parking stands with stairs? YES
OK, there seems to be an issue with updates on FSDT side.
I had the last version from the FSDT Live Update, with the PMDG B738 perfect compatibility. And then, I had no pax in the Jetways but also not those I added with the Editor.
Following your advise, I ran the new Addon Manager exe and now, I have no more B738 compatibility, but I have the pax back!
So there is a glitch somewhere.
BTW, thank you for helping Umberto during this busy time!