While "R2" releases are generally considered more stable than the initial ".0" releases, the 15.1R2 build had a mixed reputation during its prime.
Have you encountered a "REN 2 Link" error on your Juniper gear? Share your show log messages output in the comments below. juniper ren 2 link
Start with the basics: check the physical and logical status of the interfaces. While "R2" releases are generally considered more stable