I think if the human body were to reach the point where the rectum 'froze shut', not being able to defecate would be the least of your problems. Hypothermia would be a much more likely killed than sepsis. I've never come across this topic in the literature I've reviewed concerning the Eastern Front (presumably in WW2), and since a citation was not forthcoming I am inclined to think this is bunk.

I think I recall Alan Clark mentioning in his book Barbarossa that some German troops died of 'anal coagulation', but I thought the issue wasn't they their anus froze shut so much as they had diarrhea/dysentery which froze. Like Juvenalis says, the freezing part is probably not that relevant - when it gets cold enough for his to happen or you're that sick in a war zone, you're already in a lot of trouble.