Literally the only mod I changed to make this happen was Tinker's construct. Disabling it in the mod loader prevented this crash, why?
Edit: It's one of the adons for it. Troubleshooting now, but when I figure out, it'll be posted here.
Short answer: "Base metals" in conjunction with it.
You could post the crash report.
"We live on a placid island of ignorance in the midst of black seas of infinity, and it was not meant that we should voyage far."