I'm a client joined server game while . then i go down cave this bug suddenly come out.

Hello!

Same with mine, I went down to caves and my game crashed, happened twice. Third time, I rolled back my game and that same(sorta) error popped out

Please help! Thank you so much!

Are you playing with any client or server mods enabled? If so then please disable them all, then verify your game files via steam, and then try again.

If the problem continues, please attach the server_log.txt from the Master and Caves folders for this world. There is a button on the world selection screen that will open the folder.