@bebbi the login_required is returned when the Auth0 session cookie has expired or is not present. So, my first thought is what is happening to the cookie. All the checkSession method does is open an iframe and perform the redirect to /authorize as you show above.
Any chance you can attach a HAR file of the entire initial login and the call the checkSession? How are you initially logging the user in? ARe you using cross-origin or Implicit Grant flow? Without authenticating a user in this way no session will be established. If you are using cross-origin flow then it could be possible the cookie is not being saved due to 3rd party cookies.
@bebbi I am not sure of anything that explicitly says checkSession uses cookies, but we have this community question that provides some docs. The checkSession requires a session and sessions require a cookie so this is a bit more implicit.
Yes, please feel free to send me the HAR in DM. One more question, how are you authenticating your users? Are you using a social connection like google?