More precisely, you are using a customization profile made with a very early version (Sept. 2018, I believe) of GSX L2, which had a bug in the jetway logo selection. The bug has been long fixed but, if you made your customization before the fix, your .INI file is bugged.
As you can see from your log:
Loading user customizations from C:\Program Files\Lockheed Martin\Prepar3D v4\Flightbeam\Flightbeam - KDEN\scenery\kden-flightbeam.ini (file is designer-provided and won't be modified in-place)
Loading user customizations from C:\Users\Fred\AppData\Roaming\Virtuali/GSX\kden-vc5jwv.ini
The scenery already comes with its own GSX customization file so, you might not need one, and can safely remove yours, which has an higher priority over the one provided by the scenery designer and having a problem, is causing the error.