Happened again after recruiting Sonia. "Failed to load img/pictures/Angel_5.png".
Copied angel_3.rpgmvp as angel_5.rpgvmp and it worked again.
Also during her introduction scene at the Mansion, it kept complaining that it failed to load People3_6.png, but only blocked the game afterwards.
A copy of Sonia_1.rpgmvp to People3_6.png fixed that.
Edit: ...which is quite funny since it seems that was supposed to be the head butler. Hahahaaha....
Edit2: Visiting upstairs, same problem now with "Maid_1.png" and then "Maid_5.png"