I upgraded to Adobe Digital Editions 2.0 from 1.7 which I use to borrow ebooks from the library. I transfer the ebooks from my computer to my Kobo ereader. Everthing was working just fine with version 1.7. Since upgrading to version 2.0 I can no longer open ebooks that I have transfered to my Kobo ereader.
I checked and Adobe Digital Edition verion 2.0 and it indicates that both my computer and Kobo reader are authorized.
ADE sometimes gets registrations confused and half registered.
Probably worth deregistering and re-registering.
To de-register the computer, ctrl-shift-D to ADE (cmd-shift-D on Mac), restart ADE, and then reregister.
That may well be enough, but if needed:
to de-register the device, ctrl-shift-E to ADE (cmd-shift-E on Mac) with the device plugged in, restart ADE, and then reregister.
There are also bugs in ADE2.0 that mean using it with some eReaders goes OK with most books, but not with Overdrive library books.
My Sony PRS-350 is an example. The only solution I found was to revert to ADE 1.7.2.
(Quite a few other bugs in ADE2.0 as well).