This solved it for me (from the discord):
One thing to try is (when MSFS is not running! ):
Start the FSDT Installer, do an update Check first.
Go in the Config page, uncheck the Seated passengers for the Fenix.
After the checkmark goes away, re-enable the checkmark.
BE SURE you click the CLOSE button in the Config page!
If you change anything in the Seated passengers window, and exit the installer with the x icon or the Alt-F4 instead of the Close button, the various layout.json files for the liveries won't be updated, so MSFS won't know of the changed files, resulting in pink textures.
Also (from the manual):
Adding new Liveries
When you add a new Livery for a supported airplane, you MUST start the FSDT Installer again and enter the GSX Config page, to let GSX modify the airplane files for the Livery variation that has been just added. The list will pause for a few seconds, showing “Seated Passengers Loading”, while it’s scanning for your installed airplanes and newly added liveries.