Was unable to open an ADE book that was down loaded to MacBook from Library and transferred to Kobo. Said not authorized and couldn't open book! So I unauthorized my kobo thinking I could try signing in again but no option to do so then downloaded 4.5 ADE but still cannot re authorize my kobo After calling both Adobe and call Kobo help desk they had me trash 4.5 but then 3.0 was not accessible and told me there is a compatiblity issue with Catalina OS help!!
I'm not sure if this approach will work for others, but I'm posting just in case it does. I'm on Mac Catalina, using Adobe Digital Editions 4.5 to manage library books on my ancient Kobo Aura. Out of the blue, I began getting the "you aren't authorized" message.
I tried all the device authorizing/reauthorizing/ADE uninstall/ADE reinstall suggestions and nothing worked UNTIL I tried deleting some of my old e-book files (stored in Documents/Digital Editions). Next I renamed the Digital Editions folder, and as suggested by another Kobo user, went to the library site and downloaded a random book I'd never downloaded before. That book opened perfectly on my Kobo, as did all the books that previously hadn't opened because of the authorization glitch.
This approach makes no logical sense to me, but it did work in my case. Hopefully it will work for others.