Thats solution works for me:
1) I go to folder where it is placed the Kindle for Pc, and renamed the file renderer-test.exe to something (Ex: renderer-test-xxx.exe)
2) Open de Kindle for Pc application and removed the laste books with the problema of conversin with calibre.
3) Force to Kindle for pc to re-donwload these books
4) When the books are already downloaded, the new books are can now be converted with Calibre.