Hopefully it’s a one-off, due to a combination of factors as Mick says.
Once the temps have recovered a little, clean and dry all the seals off. I’d do your door seals as well, while you’re there.
Get some seal lubricant (gummi-pflege) e.g. WOL Seal Lubricant - 21926 Rubber Care Tyre Door Window Boot Rubber Seals 75ml - Rubber Treatment Stick - Deep Cleaning Car Tires and Rubber Mats (1) : Amazon.co.uk: Automotive which allows the seals to remain more supple. Recommended for any rubber seal on any car really - makes a noticeable difference when the weather’s like this 
A few other mentions of it here, in a few other threads if you want a bit of context:
Let us know how you get on.
Cheers,
Steve