It might be worth creating a test user and seeing if you can reproduce the error with that account. Try going through the login process with your browser’s inspector opened to its Network tab. That might give you some details about what’s triggering the 422 error.
If you haven’t already done so, enable the verbose discourse connect logging setting on Discourse. That setting adds some additional log entries to the Discourse logs.
If you are seeing this in the Discourse logs, are there any additional details about the request shown when you click on the log entry?
There’s some information about debugging common DiscourseConnect issues here: Debug and fixing common DiscourseConnect issues. I’m not sure that it’s relevant to your case though.