Earlier this week, Apple released an update to OS X El Capitan (10.11.2). We’ve since discovered that this update breaks Kindle generation in versions of Vellum that have been downloaded from the Mac App Store (MAS).
We’re currently looking for a solution, and have informed Apple of this issue as well. If you haven’t already updated to the latest version of El Capitan, we recommend waiting until we have more information. If you have updated, however, and you are having problems generating for Kindle, we recommend migrating over to the Direct version of Vellum, which is not affected by the update.
Migrating is simple, and will not cause you to lose your purchases or your documents. Instructions can be found on the Vellum Tips Help Page.
If you have any questions, please don’t hesitate to contact us.
Update 12/18: These issues have been addressed with Vellum 1.3.3.