The 1.8 compatibility matrix is a great reference for the latest recommended code versions - 8.6(2a)SU2 is the recommended CUCM version. There are some resolved caveats listed (BFCP comes to mind). Please reference the CUCM 8.6.(2a)SU2 release notes for further resolved caveats.
These are the paths to get to each CCX logs through CLI. They may be helpful if you are having issues accessing RTMT or downloading logs through it.
If you want to download them you have to prefix "file get " and you can add one of the options (re...