Tried to run a rail duper on my forge 1.20.1 server, and it completely crashes both singleplayer and multiplayer due to a Zeta Concurrent Modification Exception. Downgrading to the 8/24 build fixed this issue.

Thought it might be the duper part but just pushing two slime blocks with a sticky piston crashes in the same way.

having the same issue

Same issue - this was added to the BMC 4 modpack a fix for this is urgent

a fix for this is urgent

Not urgent enough to post a crashlog?

Should be fixed in Zeta 29