The other plugin was better courier.esp, and this was with Kaidan 2.1.1. I'm also on Skyrim AE in case that changes anything.
Currently, I had made it not a master in sseedit, ran LOOT again and I got the same error. Then I disabled the dyndolod output mod folder in MO2 (left pane), made Kaidan a master again via sseedit, re-enabled the Dyndolod output and am running LOOT and it just finished successfully so I'm not sure why exactly it worked this time and not previously. Sorry this is really weird I spent some time yesterday working through this with someone else and it wasn't working then and nothing else has changed in my setup.
I'm also happy to test continue testing if wanted too to see if this was a weird one-off error or a something demonstrably reproducible.