We've seen this BUG again and again. When using a Boolean response from a question, after some time... something gets messed up in the Canvas UI and it starts giving a validation error, " User response is empty. Update or delete it."
This persists until you delete the question and re-create it - which is a bit troublesome, because deleting a question in the middle of your flow always seems to take some time to repair. We don't understand why this happens and we were feeling like we were spending all of our time doing this over and over again.
Our workaround now is to use "Yes/No" multiple option responses instead.
Has anybody else seen this? Can someone fix it?