After upgrading a Cisco CUPS server from 8.5 to IM&P 18.104.22.168900-1 (9.1.1SU1), I installed a second node to be used for high availability in the same cluster. Following the "Deployment Guide for IM and Presence Service on Cisco Unified Communications Manager, Release 9.1(1)", after the install I was happy to see the database status report showed good replication and no issues with any network connectivity, rhosts, etc. All looked good.
Unfortunately I then found via the System Dashboard that the new high availability server was reporting "The node is not installed or is unreachable".
The same was confirmed via the troubleshooter menu option.
Regardless, I attempted to add the high availability option in the cluster topology. The resulting error was "High Availability action failed - a mismatch detected between server versions". Odd, as both servers reported 22.214.171.124900-1 after running show version active at the CLI. It's also actually difficult to install a mismatched server version, even if you wanted to.
show os secure status
utils os secure permissive
and then rebooting the cluster eliminated all the errors and allowed the high availability feature add.