Basically what the title says. On a dedicated server only (singleplayer server is fine), sending a packet from server to client disconnects the client and puts an error in the server log (link to server log). This occurs under NeoForge, however I can't attempt to reproduce this under Fabric due to #496 (perhaps S2C packets also cause the same issue? Not sure). From what I can gather, C2S packet sends under NeoForge are totally fine.

I've made a very minimal mod that reproduces the issue, with an included build for NeoForge.