What an obscure little error.
AO3 doesn't allow authors to put HTML tags in chapter titles (they get escaped automatically), so I thought it was safe to assume the only <span> in chapter title would always be the chapter date.
But apparently the email address protect code can also appear there.
Fixed in posted
test version.
FYI, the chapter title causing the issue
will appear as "The Gunner's Dream 1.z
(supplemental;[email protected],[email protected])". There's
nothing FFF can do about that.