Hello Francesco91,
indeed a known problem:
Strange tire problem with LX rim
you might powder tube and tyre beads with talcum, sometimes is inflating above the maximum pressure helpful.
Please be careful, there is a pressure limit for the Lt rims (sorry, can’t find it at the moment).
The primary problem is that the LT rims do not really match to tyres of that extreme size;
the original 16 inch rims are slightly more wide (a few tenth mm).
Minutes ago I’ve tried to mount a Big Apple on a LT wheel and had exactly the same problem like you, but it was no problem to use these tyres on Alex Y303 (about 4 mm more wide than original rims).