Is there a way for debugging "Access Denied" errors in Drupal 8? I'm having some routing/permission issues, and I would like to be able to debug why i'm getting "Access Denied".
Install the devel module and then Web Profile module which is bundled with it. You will then see the Symfony web profiler bar below the browser screen. Click on a Request gear icon, and it will show you the route and the controller class. You can start there.
You can also search your code base for that route name and see, if there are additional cases when it is handled.