Markoz wrote on Aug 13^th, 2018 at 2:14am:

Maybe it didn't install properly, or the downloaded file is corrupt. Download the csx572_1200.exe file again (with Norton off/disabled), and reinstall with Norton off/disabled, and making sure you use "Run as adminstrator" option, and see that fixes the problem.

I think what the OP is referring to is that some variations require a different aircraft.cfg. Open up Ace 757>Select Online>Aircraft Type>757-200 RB535E4B>Variation CS 757-223 American Airlines 2018>Click Download>Variation Downloads>Click Save and you receive the error msg "no aircraft.cfg found!"  I suspect this is for future updates?