It's solved - and it's neither the Ice9 plugin nor Cakewalk 2020.09 that is causing the issue ?
It took some time to track down the culprit, but after eliminating the usual suspects; drivers, O/S updates etc etc, it turned out it was Toneboosters Compressor v4.3.2 that was the root cause of the issue on my system.
I reverted to version 4.2.1 of the compressor on Cakewalk 2020.09 and everything is working fine. I will report this issue to Toneboosters.