Multiplayer disconnections and a crash

[Jürgen Schneider Tascenion]

I run a Fabric 1.16.3 server.
While trying to join the server, sometimes I would be disconnected immediately from the server. Then this error would display in the console. The server would not crash, however, and on a subsequent attempt, I would successfully join the server.
Most recently, however, I went to the nether and jumped into the lava sea. One second or two after I died, the server crashed. After reopening the server and trying to rejoin, it crashed again. This is the crash.

[Mitchell Skaggs]

I encountered the first issue as well, but only when the server has just started up.

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I believe it occurs when the PlayerJoinCallback is called before the first ServerTickEvents.END_SERVER_TICK callback. This means that MAmbienceFabric.server is null when AccessorFabricServer.updatePlayerInstance runs, thus a NullPointerException occurs.

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I have no idea about the second one, it’s probably a really rare off-by-one error with some sound chance. I haven’t ever seen it.