So not sure exactly what was happening but I keep getting this error: https://pastebin.com/B2zDbsX6

Seems to be an issue with loading belt entities.

Update:

Just had the same issue with Large Cogwheels on a new world. I am now assuming I won't be able to continue playing with this mod as it keeps corrupting my saves.

This issue has been marked as stale because it has been inactive for 3 weeks. It will be closed if it remains inactive for another 3 weeks.

This issue has been closed since it has been inactive for 3 weeks since it was marked as stale.

As someone already stated as it might help, downgrade your Forge Version from 35.1.37 go to 35.1.36
Depending on mod management software used, on CurseForge it is just few clicks. Try it out. ;)

Optifine is thing causing the crash, removing that will also fix it