As said in the title, the caves of this server appears to have crashed partway through a regeneration due to a message with an emoticon being sent on the surface right after that finished regenerating. The caves took longer to regenerate due to some errors in the worldgen's placement of tiles in the world, and it appears that the caves shard did not have some logic loaded that it needed to process the message. Crashlog attached for caves.
Host a dedicated server, regenerate the world, and as soon as the surface finishes regenerating, send a message with a DST emoticon in it. Ensure that the caves shard is still regenerating when the surface one is already done, and you are in the server and able to send messages. I paused the caves window by "clicking" onto it to pause the terminal.
I have independently tested this and it does consistently crash, let me know if any more documentation is needed. Following log is from another test server I hosted where I sent the :eyeball: emoji while the caves was regenerating.