Tested:
- EFIdroid 02 Nov '16 (z3ntu app) + FPOpenOS 16.09 + TWRP 2.8.7 (OpenOS) : fail
- EFIdroid 01 Nov '16 (fastboot) + FPOpenOS 16.09 + TWRP 2.8.7 : fail…
Now I’m running out of ideas 
Perhaps my primary system is also important? I have LineageOS at the moment, I can try restoring an old backup of FPOpen 5.1 …
EDIT: Tested also from FPOpen 5.1 (16.08) as a primary; tested with install zip and with restoring TWRP backup; nothing worked.
Remark : After wiping LineageOS, (so without any EFIdroid) I tried to install FPOpen 5.1 with the fp2-sibon-16.09.0-ota-userdebug.zip file from the sdcard. The install seemed ok, but when I asked TWRP to reboot I got the message “No OS installed, are you sure?” and indeed nothing booted. So I installed it with the manual-userdebug.zip 's bash script that call fastboot.
Perhaps I misunderstood, but the “ota-userdebug.zip” is supposed to be stand-alone installation, right? or is a previous OS to be upgraded is required?