How to reproduce:

1. Setup your Moodle site to use an SMTP server. You can use MailCatcher or MailHog to do so.
2. As admin, go to "Site Administration > Server > HTTP" and disable "Use slash arguments".
3. As admin, go to "Site administration > Messaging > Notification settings" and enable the Email processor.
4. Create a course.
5. Create a forum in the course and make it to have a forced subscription.
6. Enrol a student and a teacher in the course.
7. Login as the student, go to " Preferences > Notification preferences" and enable Email notifications for forum.
8. Now login as the teacher and add a new topic in the forum. The topic must contain an embedded image. Make sure to enable "Send forum post notifications with no editing-time delay".
9. Run the cron.
10. Go to your mail and check that you received an email with the forum post.
11. Open the mail. Check that the image isn't seen.
12. Copy the image URL and open it in a new tab. You should see an error "Incorrect key".