Android Lollipop 5.1 AOSP - Encrypt .. does nothing, solution?

[libove]

I've had more than one AOSP based Chinese clone Android phone where the Encrypt option in Settings does nothing. (It goes through the motions, restarts the phone .. and nothing has changed).
The latest example is a Cube T8 Plus (aka Cube T8 Ultimate) tablet, running AOSP Android Lollipop 5.1.
(Kernel version 3.10.65+ [email protected] #1 Thu Feb 18 15:45:00 CST 2016, Build number T8-PLUS_V1.0_20160218).

(I've rooted the phone, SuperSU Pro v2.65, and custom Recovery TWRP v2.8.7.0 Chinese with 'Materialised' theme to get English translation).

Since setting up encryption through the normal Settings does nothing, can someone please tell me how to try encrypting another way, e.g. by command line?

thanks.

 

[libove]

Further things I've tried:
* In another thread (http://forum.xda-developers.com/showpost.php?p=63502303&postcount=41) someone suggested that some devices won't encrypt if rooted and/or in SELinux Enforcing mode, so I tried all of the following in all combinations:
** enabling SuperSU to grant all root requests during boot
** init.d to set SELinux Permissive mode
** unrooting completely
Still no encryption. (Cube T8 Plus phablet, AOSP based Android 5.1, T8-PLUS_V1.0_20160218 firmware, rooted, SuperSU BETA 2.74)