Okay, folks, if you are see the following error when you add the CUP node to your Expressway-C while implementing MRA, then you are probably not alone:
The solution is quite simple: make sure your CUP node(s) (and every other servers in your cluster, for that matter) are specified as FQDNs. Navigate to CUCM Publisher, select System -> Server and update the server names from hostnames to FQDNs by appending your domain name.
This would also ensure that the certificates obtained for your CUCM/CUP servers are trusted, as CN/SANs are going to match what’s recorded under Server Configuration.
It’s a step that is easy to forget and not many docs are mentioning this requirement (here’s the one that does: http://www.cisco.com/c/en/us/support/docs/unified-communications/unified-presence/116917-technote-certificate-00.html).