Hello Stephen, thanks again for the troubleshooting! Indeed, I am starting from the command line. What's odd, is that in looking through the log I posted, it seems to be accepting the mtoa particle plugins, but it's when it hits the xgen that it reports a warning:
C4DtoA | 00:00:00 799MB | loading plugins from /Applications/solidangle/mtoa/2018/procedurals ...
C4DtoA | 00:00:00 799MB | mtoa_ParticleInstancer_proc.dylib: particleInstancer uses Arnold 5.2.2.1
C4DtoA | 00:00:00 799MB | mtoa_ParticleVolume_proc.dylib: volume_particle uses Arnold 5.2.2.1
C4DtoA | 00:00:00 799MB | mtoa_ParticleVolume_proc.dylib: implicit_particle uses Arnold 5.2.2.1
C4DtoA | 00:00:00 799MB WARNING| unable to load dynamic library /Applications/solidangle/mtoa/2018/procedurals/xgen_procedural.dylib:
C4DtoA | 00:00:00 799MB WARNING| unable to load dynamic library
Both machines tested are using the freshest installs of MtoA and C4DtoA; Maya is also the latest (2018.5), and I've tried in both Cinema R19 and R20. The Macs are High Sierra, though I can try on a Mojave machine tomorrow if it helps. I'm not sure if it makes a difference, but the only other thing worth noting is that our Arnold's are floating and run through a license server, just in case that changes any of this.
I'll bring in my PC laptop this week as well and try rendering from it, just to see if I have more luck there.