sci-libs/mumps-4.10.0 does not work with sci-libs/scotch-6.0.0 or greater. The recent stabilization of sci-libs/scotch-6.0.0 breaks sci-libs/mumps with useflag "scotch". A world update will pull in the incompatible sci-libs/scotch-6.0.0. The ebuild does compile without any problem, but software using MUMPS with scotch ordering will fail at run time. The error is documented on MUMPS's webpage: http://mumps.enseeiht.fr/index.php?page=faq#5 . Note that the entry has been updated for MUMPS 5.0, MUMPS 4.10.0 does not work with SCOTCH-6.x at all. The sci-libs/mumps-4.10.0 ebuild should depend on sci-libs/scotch-5.1.12b only and not on sci-libs/scotch.
Fixed in mumps-4.10.0-r1.