Ok, I have found a way to reliably recreate this. It's somehow tied specifically to the STMMaskableGraphic class, as when I disable it on the SuperTextMesh object,the error no longer pops. Here are the steps:
1) Start up a STM instance with the same configuration as I have above (although the Shadow doesn't seem to matter, just the STMMaskableGraphic).
2) Set the text to "<e=excl=open><f=Retron2000Regular>Hi hi!<e=excl=close> So great it is you get to meet great master of trade, Din' K'blapp (THAT'S ME). Gots all my nice stuff an' maybe you help get more?<e=excl=open> Shiny coin for hard workers!<e=excl=close></f>" (obv the font name should be something you've got)
3) Set the text to "<f=Retron2000Regular>TODO</f>"
The exception should then pop. I included the exact strings I used in case they had any effect. The callbacks I have in there only change game speech volume (not for any sounds handled by STM incidentally, so no possibility of interaction there).