Problems with Adobe digital Editions 4.5 with Nook from my MacBook Air running OS Big Sur

Until just recently I've had no problems using my Adobe Digital Edition 4.5 application to capture books downloaded from my library and then to add them to my Nook. I have a MacBook Air running OS Big Sur.  A book will show on my Nook but when I click to open it I receive a message:  ERROR Book has been returned."  I've tested by downloading different ebooks  from the library and then immediately trying to open them on my nook, but receive the same error message. Can anyone advise why this should suddenly be happening and how I can corect the problem?  As usual thanks very much. Ross0D44