Melabonbon,
parts of your descriptions remind me of a problem I had myself with the Kobo Touch two weeks ago. Fortunately, it returned to normal by itself the next day (cf.
link).
Edit: Please ignore the following. I overlooked the previous two posts.
Did you try as Mrs_Often recommanded? Might be a little tiresome indeed. If the culprit is really one of the books, it should be one of those you put on the device later than during the first month or so, since the problem started later, right? So concentrating on those books that you put only recenty on the device might help you in finding the culprit quicker. From your posts it is not clear to me whether all your DRM books give the ADE error or only some. If it is only some, maybe you can recognize a pattern. I hope you get the device finally working.