May be if I uninstall 8.3.1 then try a clean install right from 8.5?
Unfortunately, that's not an option unless you ordered the full set of disks. You'd just have to re-install 8.3, because the 8.5 download is just an update, not a complete release.
Most installation programs open a log file that can be used post-mortem to diagnose setup problems. However, I have not been able to locate any such file for the 8.5 install. I'd suggest opening a support ticket with CW.
In the meantime, can you repeat the setup and get a screenshot of the error message to post here?