It has indeed been mentioned elsewhere. Peter Schrijvers reports this phenomenon in his book The Crash of Ruin: American Combat Soldiers in Europe During World War II. He stops short of identifying it as any one thing, but offers several possibilities. It may have been their sweat-soaked leather field equipment. Or, possibly, the vile ersatz tobacco cigarettes they often smoked. Another explanation is simple body odor. Soldiers in the field very frequently went weeks or even months without a proper bath or even a change of clothes, all the while living in the ground, urinating and defecating under unsanitary (to say the least) conditions, handling dead and wounded men, dealing with animals, etc. This, of course, was a problem that confronted American combat soldiers as much as Germans, but perhaps as the losing, retreating side, the Germans were particularly unable to tend to their personal hygiene.

Another possibility, and this is my own supposition, is that they smelled of horses. Unlike the Americans and British, the German army was dependent on horse transport down to the platoon level, where they towed the platoon's baggage cart. Sweaty horses have a unique smell, and men who handle them and do not bathe or wash their clothes acquire a particular musty odor.