I was running a slightly changed version of ragnamod 6 which natively includes rubidium. Upon world startup or joining a server with the same mod list the game would immediately crash. I switched to roadrunner and the game ran fine. I’ve now verified that the mod that was causing this incompatibility was Chaos Awakens. The exact error I was getting seemed to be caused by a script left hanging by the mod. I’m afk but I’ll update this report with the exact error description later.

Ragnamod VI doesn't have Chaos Awakens unless the slightly changed version adds it or you added it, but also Chaos Awakens works fine with Rubidium.

You got the crash report yet?