Still worth a shot checking the database name with the command-line mobi2mobi, no? I just downloaded one of my newest Amazon purchases and I get the exact same error message you are getting (except that I'm still on Vista). Sure enough, the file has no database name. Giving it a database name with the command-line version of mobi2mobi makes the file then work fine with Mobi2Mobi-Gui .
By the way... I'm not sure what you're planning to do with mobi2mobi, but in case you're looking to change the clipping limit or TTS flag on DRMed Amazon azw files, that doesn't work anymore. If you change either of those records while the ebook still contains DRM, the ebook will be unreadable on your devices.
Just a heads-up, of course. You may have other needs that mobi2mobi will work fine for.