A couple of possibilities are:
1. You said "large" crate. The crate should actually be only big enough for the dog to turn around in. If it is too big, the dog still has there sleeping space and a pooping place without feeling that they are pooping where they sleep.
2. The dog needs to feel like the crate is their "den", their little haven from the world - not just a cage you put them in at night. Feed the dog in the crate,with the door open - just put her food dish inside. Give her treats in the crate. Make her think the crate is a happy place so she'll think of it as "home".
3. Change the dog's feeding time. Maybe digestion is ending in the middle of the night.
4. Exercise the dog more. It may be that pooping is happening at night because that's when the dog is relaxed. Just like humans, exercise aides digestion.
5. What is the consistency of the poop? If it is loose, the dog could have a touch of diarrhea and controls it during the day but in a relaxed state, cannot hold it. If it is very hard, it could be that the dog is constipated and needs to be relaxed to poop. Add appropriate fiber to the diet. Pumpkin and sweet potato can help a dog that has less then optimal poop. There are also prescription foods available
6. Take the dog to the vet to rule out medical issues. The vet can also help with appropriate diet.
Good luck!
Oh gosh, I forgot one! Have the dogs anal glands checked by the vet. Spaniels are particularly prone to this. If the anal glands are full, poops can be painful so it's another possibility for why the dog is only pooping once in a relaxed state.