Hi @kiek,
I have found the same as the users above. It appears that any livery that does not have " " (quotes) around the title in aircraft.cfg is not able to load in PSXT.
Is this something you would perhaps be able to fix with an update? I am not sure how easy/hard it is to do, but there are some massive AI packs out there and it would be countless days of work to individually update each livery in each aircraft.cfg to add quotes to its title.
For reference, this errors in PSXT:
title = A320 AXM
Whereas this works:
title = "A320 AXM "