With the recent 'summer update' for Train Sim World 2, also known as the 'Rush Hour update', Dovetail Games have updated the Unreal Engine which powers the game to a newer version (4.26).
Existing mods for TSW2 will have to be updated by their respective authors to be compatible with the new engine. Until this is done, mods which are distributed as a .pak file are unlikely to work with the game.
Reskins which can be installed via the Livery Manager tool are unaffected.