MythicMobsExtension Incompatibility with Purpur / Yatopia (Paper Spigot) forks.
https://github.com/elBukkit/MagicPlugin/issues/816 the "MagicPlugin" had the same problem.
logs
[14:54:37 WARN]: An error occurred, setting arrow lifespan will not work java.lang.NoSuchFieldException: ar at java.lang.Class.getDeclaredField(Class.java:2411) ~[?:?] at com.gmail.berndivader.mythicmobsext.compatibilitylib.NMSUtils.initialize(NMSUtils.java:1367) ~[?:?] at com.gmail.berndivader.mythicmobsext.NMS.NMSUtils.initialize(NMSUtils.java:69) ~[?:?] at com.gmail.berndivader.mythicmobsext.Main.onEnable(Main.java:99) ~[?:?] at org.bukkit.plugin.java.JavaPlugin.setEnabled(JavaPlugin.java:263) ~[patched_1.16.4.jar:git-Purpur-935] at org.bukkit.plugin.java.JavaPluginLoader.enablePlugin(JavaPluginLoader.java:380) ~[patched_1.16.4.jar:git-Purpur-935] at org.bukkit.plugin.SimplePluginManager.enablePlugin(SimplePluginManager.java:483) ~[patched_1.16.4.jar:git-Purpur-935] at org.bukkit.craftbukkit.v1_16_R3.CraftServer.enablePlugin(CraftServer.java:501) ~[patched_1.16.4.jar:git-Purpur-935] at org.bukkit.craftbukkit.v1_16_R3.CraftServer.enablePlugins(CraftServer.java:415) ~[patched_1.16.4.jar:git-Purpur-935] at net.minecraft.server.v1_16_R3.MinecraftServer.loadWorld(MinecraftServer.java:471) ~[patched_1.16.4.jar:git-Purpur-935] at net.minecraft.server.v1_16_R3.DedicatedServer.init(DedicatedServer.java:249) ~[patched_1.16.4.jar:git-Purpur-935] at net.minecraft.server.v1_16_R3.MinecraftServer.w(MinecraftServer.java:942) ~[patched_1.16.4.jar:git-Purpur-935] at net.minecraft.server.v1_16_R3.MinecraftServer.lambda$a$0(MinecraftServer.java:180) ~[patched_1.16.4.jar:git-Purpur-935] at java.lang.Thread.run(Thread.java:834) [?:?]