The new version of KP claims it will not accept .mobi files created by 3rd-party apps. For that reason, I will not download it until I know whether Scrivener will be able to compile and make nice with it.
It appears that Amazon has cried uncle and will eventually phase out .mobi. (good riddance). They’ve already made epub their go-to format. Yet they’ve also painted themselves into a corner—I don’t think Kindles will open epub. Yet. I doubt they can update older Kindles to be able to do this, but I’m pulling for them. Good luck with that.
But I want to compile in Scrivener and be able to open on Kindle Previewer. One would think that since Amazon is now focused on epub instead of mobi, that KP would also open epub. It doesn’t.
If Kindle Previewer is failing to open ePub files, that would be a question for Amazon support. But ePub has always been the intermediate format that Scrivener (and probably most programs) passes to Kindle Previewer (and to KindleGen before that). Amazon has always discouraged using third-party software to create .mobi files directly.