I don't use the default atc since I am using sids and stars
I haven't asked you to change the way you fly. I've asked you to TRY turning it off that program, and see what happens with the default ATC. If the default ATC can work over KLAS correctly, it means the scenery is fine.
The fatal error occurs with vox specifically at this airport
You should add "of those I've tried". You couldn't possibly have tried all 20.000+ FSX airports...
Its just a message of a fatal error in the vox box right at the time it tells me to switch to the tower for a final clearance, but everything else functions fine.
If the message comes from that program, you might be better served asking to the developers, I never saw that program running myself, don't have the slightest idea of the meaning of one of its diagnostic.