To quote a pro sound engineer I once worked with, "sometimes it's a single button that makes the difference between genius and idiot."

I wouldn't say it was a single button in this case, more a single digit. I moved to my MacBook Pro to see how I went there and had no issues at all, so I started looking at the differences between laptop and desktop, long story short, that final digit in the update/operating system. Updated the iMac and away we went, all systems go. It pays to check ALL updates across all aspects of a given task, Logic Pro, Band in a Box, audio interfaces, oh and of course the actual operating system on your computer.

I shall slink off with my tail between my legs whilst thanking those who offered advice as to how I might fix the problem.

Cheers, stay safe and stay well.

Andy