latest.log from crash: https://mclo.gs/QFLaFUj

It looks like MixinNeoForgeContraptionInteractionPacket.java is incompatible with Create 6 on 1.21:

[07Aug2025 08:47:00.305] [modloading-worker-0/FATAL] [mixin/]: Mixin apply for mod openpartiesandclaims failed openpartiesandclaims.create.neoforge.mixins.json:MixinNeoForgeContraptionInteractionPacket from mod openpartiesandclaims -> com.simibubi.create.content.contraptions.sync.ContraptionInteractionPacket: org.spongepowered.asm.mixin.transformer.throwables.InvalidMixinException @Shadow field interactionHand was not located in the target class com.simibubi.create.content.contraptions.sync.ContraptionInteractionPacket. No refMap loaded.

interactionHand appears to exist in com.simibubi.create.content.contraptions.sync.ContraptionInteractionPacket in at least on Create 0.5.1 on 1.20.1 but not on Create 6 for 1.21.1.

It looks like interactionHand is also there on Create 6 for 1.20.1: ContraptionInteractionPacket.java. Probably because in 1.21.1 ContraptionInteractionPacket is a record and it's just hand but not sure, that's a bit beyond my understanding.

too problem

Thanks for reporting!
No idea how I missed this.
You are correct, it's supposed to be "hand" on 1.21.1 instead of "interactionHand".

Please try 0.25.4.

Awesome, thank you for confirming!

Looks like that fixed it! And no crash with the other 400 or so mods in this pack either. Thanks for the quick fix!