A variant on the ubiquitous "Oops" message

I have a new version of the "oops" message problem. I can download a .acsm file from my library and open it on ADE 4.5.1 and then transfer it to my Kobo via ADE. Still on my PC, in ADE, I can click on the Kobo in ADE and open and read the book. However, now, after succesfully reading umpteen books on the Kobo, downloaded this way, when I eject the Kobo and then go to read the book, I get the "oops" message. I have not changed the hardware or software, nor any authorisations. The only thing that I can think of which is different is that I am at the moment in France, whereas normally I am in England when I download the books. I can still download books from the library on my Android device and read them in Overdrive, so I don't think this can be the problem. Anyone any ideas?