I had the problem with the micro-SD ... but anyway, did you installed the 15.1 firmware and did you go via "open with"? Adobeviewer is not the default reader for epub files.
By the way firmware 14.2 from the Ukrainian site does not have "open with" option, so no drm epubs could be opened with it.
Another idea - maybe you could try de-authorizing the device and then re-authorizing it again. Between the two actions I would delete all ADE related files in the internal memory (including the hidden files). Then you copy your book back to the device.
