None of my previously working custom GSX jetways show since the last Live Updater run
The last update cannot possibly have anything to do with you losing jetways on a 3rd party airport that comes with its own SODE jetways.
and I DO NOT get any red X's.
That means SODE *has* found the SODE jetways the scenery comes with, because you do have both a SODE XML file *and* the actual objects, so SODE is not reporting any errors.
The DSK jetways are still listed in the ini file in the GSX folder, but they do not show in scenery.
Of course they don't. If a scenery comes with their own SODE jetways, GSX won't try to create its own, even if you already have an .INI file for it. This is to prevent double jetways if a scenery that initially didn't had its SODE jetways is subsequently updated or reconfigured to have them.
If I try to add a new jetway there is no option in the GSX UI anymore to select which jetway version to use, and when clicking APPLY there is no new jetway showing.
Of course you can't. If a scenery comes with their own SODE jetways, GSX won't allow you to create new GSX ones. The ONLY way to let GSX create its own jetways, on a scenery that comes with its own SODE jetways is to
remove the SODE XML file for that scenery in the C:\ProgramData\12bpilot\XML folder.
I know you will say it is nothing to do with GSX, but could you possibly point me in the direction of what kind of thing in a scenery would cause this? What I should look for?
Of course it doesn't have anything to do with GSX. You probably removed the SODE XML file when you originally made the GSX customization, now you must have either reinstalled or updated that scenery or ran its configuration, or any other action that caused the SODE XML file the scenery came with to be back, so you forgot you have to remove it again before you can replace jetways with GSX.
And no, I assure you the Live Update cannot possibly have anything to do with this, we don't touch *any* file from other developer sceneries, that is certain.