Infinite scroll only allowed that spare tag to mess up an infinite number of your posts. These errors are cumulative, and the more posts there are, the worse things are going to be. That is why the error showed up then.
In all the instructions to fix that error, selecting "WP should correct invalidly nested XHTML" is only HALF the solution; the other half is to edit posts until the error is fixed. You didn't do that part, or the problem would have been solved way back at the beginning.