CM12 Samsung Galaxy Note 3: Unable to encrypt
I came across the same issue. It has to do with permissions. You should set SELinux to permissive before starting the encryption. To do that, access shell via adb shell or terminal emulator and type the following;

su
setenforce 0


Now if you go to about section, it will show SELinux as permissive instead of Enforcing. Now go to security -> encrypt and follow along the wizard. It will work.