Those particular errors are occurring because some content is being output before session_start is called (when the user auth is set up). Probably a different warning or notice.
You would be able to see this by looking in the Network tab of the browser dev tools. There will be a response that starts with "QA_AJAX_RESPONSE".
If you're able to consistently reproduce this, see if you can find the warning there. Also try removing all non-default plugins to see if the warning comes from one of those.