Hi,
I too am having this issue.
I’ve not had a chance to check the logs yet, but I did the same as the OP, can run Live Update fine, airport cache builds fine, etc, but within the sim nothing GSX related works.
Even choosing settings and disable GSX doesn’t do anything.
Addon Manager loads fine and says GSX is activated etc.
Latest version, P3D 5. 3 latest on a clean install (I only just finished reinstalling everything). Windows 10, high spec machine, everything else is working fine.
It was working so I’m inclined to agree that something has been broken by this most recent update.
Hope we can get an answer / fix soon!
Gareth