Why is my pumpkin soup sour?
Hmm. I would say that pumpkins maybe have a slight bit of acidity, but I would never think of them as distinctly sour. I suppose it could be the variety you are using. Also, taste your stock by itself to make sure it isn't contributing an unwanted sour note. All of that said, it may just be one of those expectation things. We are used to pumpkin in a sweet context, and your brain wants that little bit of sugar there to match the template, so it seems sour without it.

If you really want to test this out further, try making a small batch of the same recipe with no spices and plain water to see if it is truly the pumpkin that seems sour to you.