As a follow up. I had no other CnC apps open. The error occurred from a PC boot from off.
After a reboot I get a different error message. The game exits upon clicking OK. Restarting the game reproduces the error and exit behaviour.
The download from the CNC site worked for me to get the game launched - however I'm still getting a prompt to install 8.1. The change log at Releases · CnCNet/cncnet-yr-client-package · GitHub is missing.
What should I do here? Decline the update??