How to prevent my hub body from freezing? In the winter, as soon as its gets a little bit below 0°C, after 10-15 minutes the body of my rear hub gets frozen. This means the pedals/chain do not have any grip anymore on the rear wheel, the pleasure is over, and the walking begins. Peeing over the cassette helps for another 15 minutes, seriously, but I only carry limited amounts.

I use my bike in summer and winter. And try to keep it clean with water and soap (no high-pressure). But it is stored in my garage, where temperature is just above freezing point.

Any tips from preventing it to freeze?
Winterize your freehub body.  This kind of failure is common in colder temperatures and a hub acting like this should not be ridden.  In addition to the safety issue, you can destroy your freehub with partial engagement.  The freehub needs to be disassembled, the factory grease cleaned out of it and regreased with a winter weight grease.