I thought of something that might give us some better information on what is going on. Try changing line 107 of lib/WeBWorK/ContentGenerator/LTIAdvantage.pm (which is just an end brace) to
} elsif ($c->stash->{LTIAuthenError}) { debug($c->stash->{LTIAuthenError}); }
Make sure you have debug set to 1 in the webwork2.mojolicious.yml file, and then when you get the authentication failure see what shows up in the debug log.