I never took this warning seriously.  For me it was always that I'd moved or deleted something.  True, poser will ask for textures, but I've found that once it asks for about 4 or 5 different textures from different locations that warning usually pops up.  I just ignore it and after the scene has finished loading I then go into the mat room and reload textures manaully, or select either "reload textures" or "render", both of which will sometimes begin the process of looking for textures again from where it stopped.  Once I get a render without the warning, it's time to resave.