We were having a similar problem (same message) with a Pandigital Novel. After reading the previous answers, I managed to fix the problem this way:
I turned on the e-reader, connected the USB cable to my PC, and mounted the reader's "drive" without using the Adobe software. (My PC is a Ubuntu Linux machine.)
I think there were two expired library ebooks "stuck" in the reader.
I erased these files:
Digital Editions/Annotations/Digital Editions/Sew_Serendipity_Bags.epub.annot Digital Editions/Annotations/Digital Editions/Ties_That_Bind.epub.annot
Digital Editions/Thumbnails/Digital Editions/Sew_Serendipity_Bags.epub.png
Digital Editions/Sew_Serendipity_Bags.epub
Digital Editions/Ties_That_Bind.epub
I used a text editor to examine and modify this file:
./Digital Editions/manifest.xml
The manifest file has book entries bracketed by <de:contentRecord> and </de:contentRecord>. I saved a copy of the file and then removed the two expired books. I also cleaned out duplicate entries. (The free books were listed twice.)
After saving the file back to the ereader, I disconnected the USB cable and checked the bookshelves. The Adobe bookshelf was now empty. The Kobo bookshelf had the free books plus the two I had tried to remove. A restart might have cleared this, but I used the Kobo bookshelf management to make them go away there, too. I then shut the reader down.
After bringing the reader back up and attaching it to the Windows PC Adobe Editions, we were again able to get books from the library and put them on the reader.