SM-T815: After flash custom ROM, bootloop with "kernel not seandroid enforcing"

Hi,

I tried to flash a custom ROM this morning and everything went peachy until it I rebooted. I was presented with the Samsung boot screen and in red writing at the top it said Kernel not seandroid enforcing. I know that this by itself doesn't mean the end of the world, as it's just a security message. However, the bootloops that accompanied it were/ are more of an issue.

This is what I've tried to rectify the issue thus far:
Different versions of TWRP (all compatible with T815 and android version, obviously)
Different ROMs (RR & CM)
Installing permissive MM knl after MM ROM.
Re-downloading ROM and recoveries
Flashing CM recovery (this almost soft-bricked me!)

I'm doing this all via TWRP on the tablet, apart from one flash of a TWRP via Odin.

My device is of course rooted and has been for a while, and I am able to access recovery with no issue. It's on 5.0.2

I've kinda run out of options now. I consider myself to be a reasonably seasoned flashaholic, but this is my first Samsung device and I'm getting the feeling that I might be missing something obvious.

I hope someone can help. I'm tired of Touch Wiz - I want my AOSP!!!

Thanks in advance.

Mike