Modpack: ATM 10 1.15

Stalactite proced and hit another player, causing a server crash.

Fixed by deleting the mod and rarcompat and lauching the server again. The launch sequence skipped the stalactite entities and no crash occured.

Crash log: https://pastebin.com/xh564Kbs

seems to be caused by https://www.curseforge.com/minecraft/mc-mods/corail-tombstone

This code snippet to be exact

public static ServerPlayer getPlayerForLogin(MinecraftServer server, GameProfile profil) {
    return server.getPlayerList().getPlayerForLogin(profil, (ClientInformation)null);
}

vanilla calls this.updateOptions(clientInformation) in the ServerPlayer constructor
and in there this call occurs this.language = clientInformation.language()

which causes the exception you see in that crash log

@SSKirillSS @SiverDX I added a default ClientInformation to that method to 1.21.1 version for the next release (it was time to change it), but this method is only used when a 'Receptacle of Familiar" tries to capture a tamable entity having a player owner not being present on the server.
So, it doesn't correspond to the @qochwip explanation about "stalactite" entities hitting a player (maybe the wolf in the log).