Processing the attached document with luatex, I get I guess we're talking about U+00A7, the section sign, \S in plain.tex?
I suppose something somewhere is overwriting that definition, and we should \let the plain \S as some other longer name to avoid the problem. (Plain LuaTeX itself does not redefine \S, but I can easily imagine it might happen somewhere along the way.) Of course, that should really be done for all such plain control sequences used "internally", but life being what it is ... -k
