working with tcp sockets and can be causing this myself but it is not clear what is causing it:

[Fri Jul 22 20:03:14 2022] Welcome to Vendetta Online.

vo/vo_servernotify.lua:211: attempt to call global 'SendQuestProof' (a nil value)
stack traceback:
vo/vo_servernotify.lua:211: in function 'func'
vo/vo_servernotify.lua:2361: in function <vo/vo_servernotify.lua:2357>
vo/vo_servernotify.lua:211: attempt to call global 'SendQuestProof' (a nil value)
stack traceback:
vo/vo_servernotify.lua:211: in function 'func'
vo/vo_servernotify.lua:2361: in function <vo/vo_servernotify.lua:2357>[Fri Jul 22 20:03:14 2022] Welcome to Vendetta Online.

happens after ReloadInterface() sometimes when I fixed or tried to fix an error in my code and reload

I never fully debugged this but it's related to issues with plugins that somehow prevent some of the game's lua files to get reloaded. I found that reloading the interface after a logout was a safe way to do it, but for me the issue also went away after I was done debugging all third-party plugins that I'm using (but I don't what exactly I changed in them to address this or even which one exactly was the culprit).