Just recently we've noticed an issue where accounts are not able to be added to FileVault. I have a case open with Apple at the moment, but wanted to see if anyone here had noticed this issue.
The problem happens when FileVault is already enabled for a user and you try to enable other users from System Preferences. The users password is accepted, but when you click "done" you are immediately presented this message.
I've also attempted to enable the user through Terminal to no prevail.