Device authorization - too many machines but only using one!

I  have a Kobo eReader that I'm using on a MacBook Air (Mac OS X 10.11.6) to try and sync digital books from my local library. I consistently have problems accessing the files and have had to reset the Kobo, reset the authorization on Adobe Digital Editions (V 4.5.1) and reinstall the books.

I'm now getting an error message that says "Limit of number of machines allowed per ID Reached" (see attached screenshot).but I've only used one Adobe ID and one device. Tech support was unhelpful and said I should post my problem here.

Please help.

Thanks!

becca