Cool input, thanks a lot. 
But did not work for me. I found another solution that works for me.
Destroy the LUKS Header for Filesystem Encryption
- You need a running recovery (TWRP) which will allow you to drop to a shell as root.
- Locate the userdata partition by looking around in /dev/block/platform/*/by-name/userdata.
- Write out a ton of zeroes to the whole partition to wipe the LUKS header:
FP2# dd if=/dev/zero of=$PATH_TO_YOUR_USERDATA
- Format /data partition with TWRP
- Reboot and start encryption again.
- Notice: fastboot erase did not help in this case.