The MobilieRead Wiki has FAQs for the Kobo and offers the this solution: (Unfortunately the Kobo site's FAQs page is not functioning.)The content you are trying to view is locked. Please use your adobe ID to authorize your ereader with the Kobo desktop application or Adobe digital editions.
How to deauthorize your Nook? Ctrl-Shift-E (Command-Shift-E on a Mac)
And to deauthorize your computer of your Adobe ID: Ctrl-Shift-D (Command-Shift-D on a Mac).
I think you'll have to do both in this order.
Leave a comment if this solution worked or didn't work for you! If it did not work, please contact your local library and ask them to send the problem to OverDrive's technical support team.