Lastly, I've stumbled upon an error I did not expect. I redirected users inside my controller, based on some conditions. But for a strange reason, the redirects did not always work. Here is how to prevent things like that. Add a "options: no_cache" parameter to your mymodule.routing.yml to prevent the controller from being cached: 

mymodule.dashboard:
  path: '/dashboard'
  defaults:
    _controller: '\Drupal\mymodule\Controller\Dashboard::render'
    _title: 'Dashboard'
  requirements:
    _role: 'authenticated'
  options:
    no_cache: 'TRUE'