This device has booted from the backup JunOS Image

(2017-01-25)

Problem

WARNING: THIS DEVICE HAS BOOTED FROM THE BACKUP JUNOS IMAGE
It is possible that the primary copy of JUNOS failed to boot up
properly, and so this device has booted from the backup copy.
Please re-install JUNOS to recover the primary copy in case
it has been corrupted.

Solution:

root@ex2200> request system snapshot slice alternate
[...]