I read that thread, and the possible explanation made by Nick surely doesn't apply to our Addon Manager software which surely doesn't write any airplane configuration files (either add-on.xml or aircraft.cfg), and surely nothing in the Documents/A2A folder which is the one A2A seems to be unable to write to.
GSX surely *reads* the aircraft.cfg, but it never tries to write to it, which could possibly cause write permission issues like that. And, in general, neither GSX or the Addon Manager ever write to ANY files not belonging to our own folders under %APPDATA%\Virtuali or %PROGRAMDATA%\Virtuali
I'll try to contact A2A to see if they have any ideas about this.