I got this crash from someone else who made a PO3 server: https://pastebin.com/BtTpaXzj

Using Chance Cubes Version: 1.12.2-4.0.0.268 Beta

I have this same problem, does anyone have a solution?

Can confirm; FML 2815 for 1.12.2 + only the Chance Cube mod results in the same error.
For PO3 users, I'd recommend either downgrading to build 3.0.38, or, disabling Chance Cubes server and client side until a resolution is found.

Fix was just uploaded to curse. Let me know if there are still issues!