I'm getting the same message. As of yesterday, my books have changed position on my Bookshelf without my making any changes to any of my book texts or accounts. My sales-rankings for two of my books have been switched, on their book pages, such that the (much) slower selling book is getting the ranking appropriate for the higher selling (and vice-versa). This after there had been no updates to my sales-rankings for a couple of weeks. I sure hope this is something that is going to be back to normal as soon as possible and that it is not masking new sales.