Oh, it seems with the latest fix there was a new bug introduced, cf. yrcmex10 problem AFTER December 5, 2012 (see especially Ulrike Fischer’s comments). Obviously now the file
yrcmex10.mf is missing. This file is included in TeX Live, but it’s obscure, where they got it from, because it is not part of both ZIP files (linked in “commands of "yhmath" do not work in MiKTeX”). Extracting from an installed TeX Live or from http://mirror.ctan.org/systems/texlive/tlnet/archive/yhmath.tar.xz should help.