I have lately been getting this error when anyone joins the server:

https://paste.gg/p/anonymous/0ea0d46978e74feda115df8e2914234f

I have also generated a plugin dump of it:
https://dump.viaversion.com/fffeda1282e8830095f36273cb58b52990b14b05f1730d6e8b62c6f6a7081f66

The server version is paper spigot 1.15.2
and I'm using a proxy, the proxy is bungeecord 1.16

The console told me to make a bug report of this issue