mobi not working anymore?

hello all,

a bit of a weird situation. whenever i import books in caliber, it converts them automatically to mobi and then i send to my kindle paperwhite. since a few weeks, all those mobi books will not open on my kindle, i get the message "could not open book. if it was purchased on amazon, try to redownload the book".


but the funny thing is that all my older mobi books already on my kindle still open no problem. only new ones that i add dont open.

if i convert them to azw3 they work.

any ideas? running mac with latest 4.3 calibre... thanks