As said already by Philip, all you need is some Gummi-Pflege and and apply to the door/window rubbers. With the product applied they’ll remain supple. I’ve also used it in Alpine and Canadian winters with no issue.
e.g. https://www.amazon.co.uk/Liqui-Moly-7182-Rubber-Care-75-ml/dp/B00CSYM1K8
I’ve used this on cars for nearly 30yrs, when I saw this exact issue with an E36 BMW 3-Series which had frameless doors. Before using this product I’d need to keep a small metal ruler on-hand to run around the door frame to get it to open!