I'm new on this community. I tried to leave a comment a few minutes ago on the page "section numbering in 'tufte-book'", but i accidentally post an "answer".
The given MWE uses
tufte-book. I suspect that
tufte-book uses the package
titlesec. The most recent version of
titlesec, which is a week old, causes the same error.
On "Formatting subsections and chapters in tufte-book" is stated "...The tufte documentclass loads the titlesec package..."
So this makes me believe that
titlesec is the cause of the problem, even if this package isn't visible in the shown MWE.
I wasn't able to rectify my post, because it was deleted. I now have to have 50 point, but i don't know how to get them.