I would put money on the reaction being exothermic. The epoxide ring is high strained configuration and and therefore would be expected to release a significant amount of energy upon opening. The other reactant is generally an amine and hydrogen can be made to fall off them quite easily normally (fall off being used in the technical sense here ;-) ).

From a chemistry point of view temperature generally only affects the speed of the reaction (within reason, you obviously get different products if the reaction catches fire). There are reactions where the end products you get are dependent on the temperature of the reaction but they aren't very common and I don't think that would be the case here as this chemistry is "aggressive". My guess is that this is more of a mechanical issue. To produce a strong end product the amine hardner needs to be able to cross link the epoxy. If the mixture is cold the molecules won't be very mobile so it's less likely the amine will bump into and be able to link to more than one epoxy leading to a product that is mechanically quite weak. The lower the temperature the worse this would be.