I wrote that one, I think that rule was added in the most recent rule set update as I added a bunch of MCA-related ones.
IIRC, MCA v5.2 doesn't have the levelled list item "MCA_ashlander_scarf_random" defined in it. So, when you meet either of the NPCs "elder_ashlander_f1" or "elder_ashlander_m1" the game'll throw an error.
You can edit the MCA Addon in the CS. Ensure you select MCA.esm and set the addon as the active file. Open up the two NPC entries and delete the "MCA_ashlander_scarf_random" item from their inventories.