Aside from the user_featurization issue, I am having a similar problem and wrote about it in this thread here: "There was an error loading messages and polling has been stopped."
My Rasa version is 2.4.0 and Rasa-X is 0.38.1 which are compatible.
@RGK do you have other instances where the conversations are not affected and the since parameter is set to a number other than 0?