I had this problem on my Kobo Glo after upgrading to 2.4 - so it doesn't seem to be just a Kobo Touch issue. Adobe Digital Editions seemed to think my Kobo was still authorised, but deleting the affected epubs and recopying them using ADE didn't seem to work.
I solved it by adapting the instructions above (1) attach the Kobo to the PC and tap connect (2) delete the .adobe-digital-editions folder on the Kobo (3) reboot the Kobo, otherwise ADE can't see it (4) authorise the Kobo in ADE.
I could now read the books on my device, but the covers were just black rectangles. So I deleted the books and reloaded them using ADE and everything was fine.
I'd add that the reason I bought a Kobo rather than a Kindle was precisely because Kobo does not tie you to a single bookstore - despite the fact that I've bought most of my ebooks from Kobo - so I hope this problem does not reoccur.