4 years ago my father took out a loan to buy his house. The bank then registered a mortgage on the property. We recently had him placed under curatorship. We've just learned that he hasn't paid the last instalments and that the bank is threatening to...

...seize his house. Is this possible despite the protective measure?

Yes. The protection of the protected person's home has the effect of limiting the acts that his representative can perform (art. 426 C. civ.). However, this does not mean that the property cannot be seized by creditors, in this case the bank (CA Paris 18 sept. 2001). The bank will be able to seize your father's house without having to seek authorization from the guardianship judge.