you might want to enable debugging to troubleshoot
Debugging your DiscourseConnect provider
To assist in debugging DiscourseConnect you may enable the site setting verbose_discourse_connect_logging . By enabling that site setting rich diagnostics will show up in YOURSITE.com/logs . Be sure to 
the warnings box at the bottom of YOURSITE.com/logs .
reference: DiscourseConnect - Official Single-Sign-On for Discourse (sso)