https://github.com/Vazkii/Psi/blob/14b3a866ffca67dcfac02fcde73cc764d38590e2/src/main/java/vazkii/psi/client/core/proxy/ClientProxy.java#L98
ObfuscationReflectionHelper#getPrivateValue needs the unmapped/SRG name, not the mapped name. Noticed the error in the client log on ForgeCraft 1.

I'm surprised it only blew up now, a temporary fix has been released, but a move to mixin is probably happening