It did get very hot. I am in Copenhagen, Denmark. We had a very hot summer, day temperatures around 30 degrees in, and the Ivy has been in my windowsill exposed to midday sun, the hottest of its kind.
So I might have overwatered it before I left, and then it got cooked while I was away. Sounds very reasonable. It is still moist to wet in the soil, and I've only watered it once after I came back.
I have now placed it out of its pot, and in the light but out of the sun. I truly appreciate your answer, I hope it comes back.