Hi Gilles,
that's odd, there should be a mca_pmix_pmix3x.so (assuming you built with the internal pmix)
Ah, I didn't -- I linked against the latest git pmix; here's the configure line:
./configure --prefix=/usr/local --with-pmix=/usr/local --with-slurm --without-tm --without-moab --without-singularity --without-fca --without-hcoll --without-ime --without-lustre --without-psm --without-psm2 --without-mxm --with-gnu-ld --enable-debug
I'll try nuking the install again and configuring it to use internal pmix. Cheers, Andrej