Fixed by Autodesk technical call who uninstalled 2021.1 and reinstalled 2021.0, apparently some glitch with 2020.1 working with High Sierra OSX. Been advised not to apply the latest 2021.1 fixes.

Will probably look at upgrading my machine and OS soon, it's  a 10 year old machine and although heavily upgraded, powerful and perfectly fine, the hardware will no longer be supported by Apple on their new operating systems.

Thanks for your help / ideas.