My server suddenly won't launch - deprecated mod issue (Bug Tracker)?

This is probably intentional. It's been a while since the update that caused this crash. You might try opening your world save file with a text editor and removing all instances of "loadstring" calls within it—this should allow the save to load properly. However, if the mod you're using hasn't been updated to fix this issue, you may need to perform this operation every time you start that particular save.

I can't find it in any of the files inside the save folder. Do you know which one would need to be edited?

You can try editing the file session/02CA05C4000EE02A/0000000439 from your overworld save folder using a text editor (do not use the default Windows Notepad; instead, use a third-party editor like Notepad2). Be sure to back up the file before making any changes. Try adding the following line at the beginning of the file:  
local loadstring = function() end
I don't have access to your actual save file, but based on my experience, this should work.

BTW: The path session/02CA05C4000EE02A/0000000439 comes from the error message in your master_server_log.txt. The number 0000000439 is the name of the latest save file, and this number increases with each world save.

Edited August 7, 2025 by Whateverr

Oh wow, that actually worked. Thank you! And for explaining what the files are. That's good to know, too!