I am running a private server for myself and a friend.

crash report: https://pastebin.com/z8AXuPN0
log: https://pastebin.com/xY4uJz1Q

Quark is running on the latest version.

I created a new server today, and it has been running fine while I was playing on it alone. However, as soon as the other player tried joining for the first time, the entire server would crash. I restarted the server several times and the crashes happened every time he joined.
I was told that there is some kind of issues with advancements caused by quark?

Try disabling Auto Recipe Unlock in the quark config.