It was usually something to do with the footnote before.
So for the one you describe take a close look at footnote 21 in text mode. Check that there is no extra code that has randomly appeared (if you cut and pasted from somewhere else it might be that – so remove extra code).
Otherwise sometimes removing/rewording/shortening worked. Others, I had to remove italics – but that didn’t work all the time, and you don’t seem to have italics. The problem I found was that the ‘solution’ was just as random as when the out of order numbering appears. There was no ‘one’ fix – and fixing one doesn’t solve the problem.
I imagine it’s a bug somewhere in the plugin.
Maybe someone else has a better idea???
Good luck!