Does it also give errors and lose data when you are in the markdown editor, or is it just in the richtext editor?
There are a handful of plugins that will not work in the richtext editor, and this appears to be one of them.
Edit: I just installed it to try it out and am not able to repro, even using the richtext editor. I was not attempting to edit together with another user, but I was able to enable shared edits and edit the post, and see the changes appearing in real time. What are the repro steps to getting 500 errors and losing data?