If you'd substituted with squash, would it take care of the bitterness?
Yes I believe so. I make butternut squash soup often enough and I never find it to be bitter.
The temperature scales on the ovens were similar but our oven in Canada is larger and heats up quicker and can be set to higher temperatures. In France I cooked everything at 240C and it took forever to cook anything. I usually set the oven to 175C here at home in Canada and things cook faster and will burn if I'm not careful. The oven I had in France was smaller and a much cheaper model - an Ikea brand I think. So it just wasn't as good