Before anybody else suggest it, I have already posted this question in FSDT Forum and have got no answer, hence hoping that somebody may be able to shed some light in this issue.
For some inexplicable reason, my GSX has gone pear-shaped. Tried deleting the Virtuali folders from relevant directories; tried deleting the Scenery Index and Facilities, and then re-installed GSX but I keep getting an error message something about patched vehicles_airport restoring from backup, and then when I attempt to ask for GSX Services, back to same error about 'Could not find valid Afcad data at this position, services are not available ' that flags up at every single airport even though Addon Manager and Couatl is showing in the drop-down menu except the 'customise' option for the relevant airport.