For the person who started the thread: it's not useful to say "MM-related UI error popup" without saying what the error actually says.
See section 3.17.3 here:
https://esouimods.github.io/3-master_merchant.html#Luaerrors
Actually, it only worked for that session. Getting the UI error and when I dismiss it I can't use menus at all. So it's worse now...