Authorization Problems

I have recently switched computers... from Windows to a MAC. On my old computer I had Adobe Digital Editions 3.0 and I have now downloaded ADE 4.5 on my MAC.  The problem is that even though I can see library books on my eReader,  I keep getting a message that my eReader is not authorized to open the book... even though in Settings I can clearly see that the eReader is authorized.  I have removed ADE 3.0 from my old computer - prior to removal, I deauthorized that computer.  I have reinstalled ADE 4.5 on my MAC and even installed ADE 3.0.  I have authorized and deauthorized and then re-authorized again, rebooted - returned the book and re-borrowed, re-loaded etc.  But nothing works.  I can open the book on my computer.  I've added the Libby app to my phone... so it's not a question of whether I properly borrowed the book.  but it will not allow me to open on Kobo.  Very frustrating.  Any suggestions would be welcome.