Doesn't surprise me. At 120V, the two resistors dissipate 0.24W each. At 240V, they dissipate 0.96W each. That's much hotter, and honestly, pretty close to their 1W rating.
When running on 240V, I would replace the two 15K 1W resistors with two 30K 1W resistors, Mouser part 294-30K-RC. They will dissipate 0.48W then; a much better margin.
/mike