I have also been having errors thrown at me every few seconds, it started happening shortly after I started having sync issues, then weird stuff started happening, duplicating items (I would pick ammo from a container, had to relog and I still had the ammo and the container also), losing items (I would store stuff in a container, had to relog and the container was like it was before I put anything there but I don't have them in my inventory).
Here is the repeating error from the logs:
[24-12-21 02:53:02.622] ERROR: General , 1640325182622> 67,664,174> DebugLogStream.printException> Stack trace:.
[24-12-21 02:53:05.614] ERROR: General , 1640325185614> 67,666,730> java.lang.NullPointerException: Cannot read field "square" because "<local7>" is null.
[24-12-21 02:53:05.615] ERROR: General , 1640325185615> 67,666,731> at zombie.network.GameClient.sendAddedRemovedItems(GameClient.java:4811).
[24-12-21 02:53:05.615] ERROR: General , 1640325185615> 67,666,731> at zombie.network.GameClient.update(GameClient.java:434).
[24-12-21 02:53:05.615] ERROR: General , 1640325185615> 67,666,731> at zombie.GameWindow.logic(GameWindow.java:223).
[24-12-21 02:53:05.615] ERROR: General , 1640325185615> 67,666,731> at zombie.core.profiling.AbstractPerformanceProfileProbe.invokeAndMeasure(AbstractPerformanceProfileProbe.java:71).
[24-12-21 02:53:05.616] ERROR: General , 1640325185616> 67,666,731> at zombie.GameWindow.frameStep(GameWindow.java:745).
[24-12-21 02:53:05.616] ERROR: General , 1640325185616> 67,666,731> at zombie.GameWindow.run_ez(GameWindow.java:661).
[24-12-21 02:53:05.616] ERROR: General , 1640325185616> 67,666,732> at zombie.GameWindow.mainThread(GameWindow.java:475).
[24-12-21 02:53:05.616] ERROR: General , 1640325185616> 67,666,732> at java.base/java.lang.Thread.run(Unknown Source).
[24-12-21 02:53:05.616] ERROR: General , 1640325185616> 67,666,732> ExceptionLogger.logException> Exception thrown java.lang.NullPointerException: Cannot read field "square" because "<local7>" is null at GameClient.sendAddedRemovedItems line:4811..
EDIT: I should add that the server is completely vanilla, and I have VOIP turned off.