Constantly switching firmware is generaly a BAD idea. You will wear the internal flash quickly by doing this, or get the risk to brick the device.
I don't recommands to use the 1.5/2.0 public firmware at all, especially if your device was send with a newer version (the build is the best information, better than firmware version)
What is the start of your device serial number?
Try to redownload the 2.1 firmware, it seems that the newest bookeen server tend to do bad download, and it's look like that the installer may not detect them...