When starting the mod with Replay Mod in 1.14.4, it doesn't even load to the menu, just says "An unexpected error occured..."

If you can provide a log, that'd be a big help with diagnosing the issue

output_log.txt

This issue is caused by the client.reduceCuboidTransformations patch. It's not likely it can ever be resolved without disabling this patch. However, I'm looking into creating a hints system to Lithium so we can detect incompatibilities like this and automatically adjust our configuration.

This patch has since been removed in Minecraft 1.15+.