This fix is for ADE 4.5. Not sure why anyone would want ADE 3.0 and not the latest software.
If you’re having trouble getting library books on a kobo it is probably a new ereader to your account, because changing things typically causes problems. Try deleting all authorizations - (on both your computer and on your ereader, but keep the ereader connected via the USB.) Check in all places to make sure it is showing no authorizations whatsoever.
Next, go to your public library website and get a NEW book - (something about trying to open a book that you already took out somewhere, causes a glitch - for example if you previously opened a book on a tablet or phone, then that’s it - it's extremely unlikely that you will be able to open it elsewhere, so make sure it is a newly borrowed book. If you are out of new borrows, because the limit is usually 10 books per library user, then use adobe to return a book, close and reopen adobe so that the return definitely happens.). Download the new book to your saving spot, and click it to make it open in adobe digital editions: Adobe will then PROMPT YOU for the authorization. Use the drag down menu to find your library’s license type: for me it is Overdrive. Then enter your (Overdrive) email address and password. This should cause the book to open AND it also should cause your connected ereader to become authorized at the same time. Click the back button at the top to get the book to close, then click your ereader, and then it’s gear sign to look to see if your ereader got automatically authorized (as it did for me). Even if the kobo is not authorized, try dragging the new book onto it. This should work immediately , or perhaps it will ask for the authorization again. You can then click on KOBOeReader in the left column (not on 'all items') to see if you can open the book as it sits in the ereader area. If it opens there, you should be good.
Michele the librarian from the previous post here (now in my own profile). Will try this, however, most of the posts on the "Oops! This document couldn't be opened." thread say that 4.5 didn't work and recommend trying 4.0 or 3.0.
Since so many people had success downgrading to version 3.0 not sure why we're having trouble with 3.0 to start. It is a new ereader as you mentioned & hasn't been opened before, so I didn't try to do a factory reset as some people suggested.
Hoping this works so I can help the customer start e-reading as soon as possible.