I found out what had been breaking the warp command (continuation of issue #263).

[Mario662629]

Funnily enough that direct packet sending method is already deprecated https://github.com/nea89o/Firmament/blob/dd6a3e3d657344f32ccf85fb112a21cd46dbd974/src/main/kotlin/util/MC.kt#L62-L70.

I am going to close this since i am not using that packet anymore, feel free to reopen if this bug persists with new versions.

Originally posted by @nea89o in #263

The error was not caused by the mod, but it's because the /warp command is overridden by a client-side command. Simply disabling this would fix the issue.