When you are attempting to rollback to r13, where did you obtain the image from? If you freshly downloaded it from Juniper SW download page, issue will persist; all available images have been resigned with the new key.
The easiest way out would be to get in touch with JTAC, get an image signed with the old key, load it on the box and then delete the key once it boots up.
Then you may continue with the upgrade plan.