Saw this error:
It looks, well, sort of serious. I mean, it doesn’t say “You got the formatting wrong” or something. Is there a ‘verbose log’ switch anywhere so I can track down the source of the error in my text (I’m assuming it’s ultimately my fault … 
I haven’t seen this one in a while, but if I recall correctly, that means you have referenced a footnote that doesn’t exist in the actual footnote list. It might be that the reference label has a typo in it, or that the formatting of the footnote is botched. I’d check on the specified line and see if there are any reference marks there.
That’s a good place to start. Thanks!