Issue now resolved with assistance of a Microsoft Support person.
The Cause IS a code change by Microsoft somewhere in Power Automate, this is an issue created by and caused by Microsoft.
I use the "Folder path" Dynamic code - this is what appears to have been changed, for me.
Created a COMPOSE action, with the following code (expression):
substring(outputs('Action_Name')?['body/{Path}'],0,sub(length(outputs('Action_Name')?['body/{Path}']),1))
This fixed the issue for now, but IF Microsoft make a change to revert back to the previous code - this will need to be removed.
For now - all is working again.
Very annoying of Microsoft to make these changes with no notice - this is unacceptable for any company, and is making the use of Power Apps, something to think seriously about