Many thanks, everything is working again!
Not sure what has caused the failed attempts - however I did notice I had created a separate usr/pwd for sftp uploads in the past - this separate usr/pwd was gone (reset to default usr name) when I checked the Plesk dashboard this morning. Might have happened during the Plesk migration?
Enfin, after I ensured usr/pwd combinations matched on server and client side I was able to update my files.
Again, many thanks, Erik