The coolant reservoir overflowed for the first time this afternoon, and the level of the coolant in the reservoir is about one-and-a-half inches above the max-fill line.

Yes it is. It could mean several things. Best case scenario you have a small coolant leak somewhere on the engine and your coolant level in the engine is low and allowing it to overheat and boil, or your cooling system is clogged somewhere. Worst case scenario you have a head gasket leak and combustion gases are blowing out through the cooling system. Check the oil if it is milky that's an indicator of a blown head gasket.