but even the books with DRM are not opening either,
Were the DRM'ed books downloaded specifically for/to the Kindle you're trying to read them on? Or were they downloaded via K4PC, then added to calibre (where the plugin does it's magic), THEN transferred to the Kindle?
If it's the latter, the recent changes will certainly cause the problems you are experiencing. If it's the former, you have an entirely different issue that you should definitely call Customer Support about.