Both crashes happened using build 18, a few builds after that, and using the latest, build 38. I did think it was Galacticraft as well interfering, especially because of this line in the first crash report I posted:
but in a separate instance with just MPS and Galacticraft installed, the mod worked fine.
I've been slowly adding the mods I am using to this separate instance, until the error returns. At the very least, I can tell everyone which mod is causing the incompatibility.