The server randomly (keep that in mind) stops responding whenever someone tries to join the server (when no one else is online, according to our perspective) (this doesn't always happen).

Restarting temporarily fixes it, but eventually, the server stops responding.

More information can be found here: https://www.spigotmc.org/threads/server-not-responding.195943/

I have never encountered this problem until ~2 days ago.