Post History

Slow cooking is characterised by a slow and low heat cooking process. For you this means that you put too much heat in the cooking process and that your food essentially (as you noticed) dried out. Basically, you took the process of grilling and prolonged it. (Grilling is a high heat, short term cooking process in which a lot of water on the outside of the food which is grilled is lost while the inside stays a at least a little bit hydrated.) So the next time you should definitely stick to low heat for slow cooking.

Regarding your kitchen tools, it'd advise to do as instructed, so keep the pressure vent open. Generally speaking, keeping the lid or a vent open or closed is primarily based on how "watery" you want your food to be. Meat and vegetables lose a lot of liquid in the process of cooking so that it might be wise to have an opportunity for the water to evaporate correctly. (With a closed pot, the water will just stay in the pot and can't evaporate.) For a soup, leaving the pot open is the exact opposite as you don't want liquid to escape.

Taking both together, your food was very hot for five hours. Hot food "produces" a lot of evaporated liquid which has to go somewhere. You used an open system so that the water escaped, leaving you with something more similar to a sauce than a soup.

You can read more about the topic on Wikipedia: https://en.wikipedia.org/wiki/Slow_cooker