More precisely, it refers to a cross-certification setup, common in large enterprises or government networks that use their own Private Certificate Authority (Private CA).
That was the clue.
Priya leaned back and typed a quick post-mortem for her team: Our machine was missing the R2R (Root-to-Root) bridge certificate for the partner’s private CA. The error message is misleading—it doesn’t mean a file named ‘R2R’ is missing. It means the chain linking our public trust to their private trust is broken. r2r root certificate is not installed