It seems like it's a problem connected to the Std_Opaque material, it's not happening if I open an old UE file created with the previous version of the importer (and of twinmotion) .
For now as a workaround I made a copy of the Old Std_Opaque and substituting it as a parent material for the new ver imported materials with the new ver seems to be stable and doesn't freeze anything.
I don't have this problem with foliage or other objects (the bark for example)