After reporting the issue I went into the enchanment window again of the weapon. Checked whether there was an ability there for DR Reduction. It wasnt but thats the same with other weapons that have inherent bonuses, so I assumed that this was normal.
After closing the enchantment window and going to battle again then (all of the sudden?) it was calculating the DR Reduction of the Arquebus normally!!??
I will let you know if this happens again and I will try to reproduce the effect that might have caused this.
Thanks again