When you have anonymous users enabled you are not able to login via the callback because of an existing session that is being detected and used in the loginhelper.
If you change line 52 of /oauthmodule/actions/custom/LoginHelper.java into
if (activeSession.getId().toString().equals(cookie) && !activeSession.getUser().isAnonymous()) {
It will ignore any anonymous user session and overwrite the anonymous user session with a fresh user session.
When you have anonymous users enabled you are not able to login via the callback because of an existing session that is being detected and used in the loginhelper.
If you change line 52 of /oauthmodule/actions/custom/LoginHelper.java into if (activeSession.getId().toString().equals(cookie) && !activeSession.getUser().isAnonymous()) { It will ignore any anonymous user session and overwrite the anonymous user session with a fresh user session.