I can't insert images to articles
My solution was simple: After updating Drupal8, ensure you run any pending scripts! Otherwise you will not be able to do certain things with your Drupal8 site.
Yesterday I updated my Drupal from some ancient version to the latest stable Drupal8. I noticed I can't add images [the forever loading problem], and I'm just a home user not behind a fancy firewall. After updating 13 pending scripts, everything was fine.
I'm not a very technical person, but I wanted to share my solution because no one else had shared this.
My website is ontu.org and thus the settings page for me was here:
http://ontu.org/admin/reports/status

Change that URL to your site and you'll quickly go to your settings page. There may be multiple issues reported on that page, so notice everything on there.