Hi,
Uninstalling and reinstalling
Calibre does not help. Go to Calibre's Preferences->Change Calibre Behavior -> Plugins and then look at the File Type plugins and hit the little triangle at the front. You should see a list of all File Type plugins installed. Highlight any you want to remove and hit the remove button (don't forget to hit Apply to make sure your changes hold). I would remove any and all K4Mobi*, K4Mac*, Mobi*, plugins you have and then install the very latest K4MobiDeDRM plugin version 3.7 (taken from Calibre_Plugins folder inside tools_v4.7).
You should then have the latest.