Thanks for your help. I hadn't found that thread. Anyway my problem is different, as P6 has no problem finding the geometry and the "visible" textures (located in the same runtime as the ones it doesn't find).

I added the P4 runtime the way the manual (and several threads here) says, going to the upper level on the library and clicking the "+". In fact I've removed and added the runtime to make sure it was correctly added.

I've tried using CorrectReference, but it keeps happening the same thing.