Unable to activate imessage even after clean install.

For the last few years, I've been unable to activate Messages on my Mac Pro. I've had repeated calls to Apple Support, and read all the postings I can find. Yesterday I once again reformatted one of my drives and installed El Capitan from scratch on that disk, and did not import settings or files from Time Machine. I did obviously authenticate with my Apple ID to iCloud to make the attempt at getting iMessage up, and it continues to give the error that it is unable, and for me to try again later. Other aspects of iCloud seem to work, an iMessage works fine on 10 other devices that come and go as I replace laptops and phones as the years pass. They all use the same iCloud account, keychain, facetime, Messages settings, and the like. Has anyone found anything so entrenched in a system that even a clean install would not clear this type of error? I suspect something being reimported from keychain or other iCloud settings, but as they exist on my other systems, I'm stumped. Currently running 10.11.2 on a Early 2009 Mac Pro with about 40TB of storage in 5 RAID arrays, etc. But this has existed for several major releases of OS X at least.