Dear Emma,
the error is not in Mrefl but in Mcool and others. As explained in 7.22.45. MATERIAL — FLUKA Manual a material with density below 0.01 is considered a gas at atmospheric pressure. If the pressure is off, you should adapt it with a MAT-PROP card.
You can find the issues e.g. by replacing the material in all ASSIGNMAT cards with BLCKHOLE, them add back the correct material one after the other, always running a few primaries.
Roman