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If Mythic has not been previously installed/configured, it fails to boot up with the error:

[22:36:56] [Server thread/INFO]: [MythicMobs] Enabling MythicMobs v5.9.0-SNAPSHOT-d0a50305
[22:36:56] [Server thread/INFO]: [MythicMobs] Loading MythicMobs for Paper (MC: 1.21.4)...
[22:36:56] [Server thread/INFO]: [MythicMobs] The server is running Paper; enabled Paper exclusive functionality
[22:36:57] [Server thread/INFO]: [PlaceholderAPI] Successfully registered internal expansion: mythic [5.0.0]
[22:36:57] [Server thread/INFO]: [MythicMobs] Mythic PlaceholderAPI Support has been enabled!
[22:36:57] [Server thread/INFO]: [MythicMobs] Mythic Vault Support has been enabled!
[22:36:57] [Server thread/INFO]: [MythicMobs] Base directory /home/container/plugins/MythicMobs/data
[22:36:57] [Server thread/INFO]: [MythicMobs] Module directory /home/container/plugins/MythicMobs/data/worlds
[22:36:58] [Server thread/INFO]: [MythicMobs] Loading Packs...
[22:36:58] [Server thread/INFO]: [MythicMobs] Loading Items...
[22:36:58] [Server thread/ERROR]: Error occurred while enabling MythicMobs v5.9.0-SNAPSHOT-d0a50305 (Is it up to date?)
java.lang.IllegalStateException: The Mythic plugin hasn't finished starting up yet, or failed to load!
    at MythicMobs-5.9.0-SNAPSHOT.jar/io.lumine.mythic.api.MythicProvider.get(MythicProvider.java:9) ~[MythicMobs-5.9.0-SNAPSHOT.jar:?]
    at MythicMobs-5.9.0-SNAPSHOT.jar/io.lumine.mythic.core.config.IOLoader.<init>(IOLoader.java:49) ~[MythicMobs-5.9.0-SNAPSHOT.jar:?]
    at MythicMobs-5.9.0-SNAPSHOT.jar/io.lumine.mythic.core.items.ItemExecutor.loadItems(ItemExecutor.java:64) ~[MythicMobs-5.9.0-SNAPSHOT.jar:?]
    at MythicMobs-5.9.0-SNAPSHOT.jar/io.lumine.mythic.core.items.ItemExecutor.load(ItemExecutor.java:49) ~[MythicMobs-5.9.0-SNAPSHOT.jar:?]
    at MythicMobs-5.9.0-SNAPSHOT.jar/io.lumine.mythic.core.config.ConfigExecutor.load(ConfigExecutor.java:453) ~[MythicMobs-5.9.0-SNAPSHOT.jar:?]
    at MythicMobs-5.9.0-SNAPSHOT.jar/io.lumine.mythic.bukkit.MythicBukkit.enable(MythicBukkit.java:185) ~[MythicMobs-5.9.0-SNAPSHOT.jar:?]
    at MythicMobs-5.9.0-SNAPSHOT.jar/io.lumine.mythic.bukkit.utils.plugin.LuminePlugin.onEnable(LuminePlugin.java:81) ~[MythicMobs-5.9.0-SNAPSHOT.jar:?]
    at org.bukkit.plugin.java.JavaPlugin.setEnabled(JavaPlugin.java:280) ~[purpur-api-1.21.4-R0.1-SNAPSHOT.jar:?]
    at io.papermc.paper.plugin.manager.PaperPluginInstanceManager.enablePlugin(PaperPluginInstanceManager.java:202) ~[purpur-1.21.4.jar:1.21.4-2412-9249a2c]
    at io.papermc.paper.plugin.manager.PaperPluginManagerImpl.enablePlugin(PaperPluginManagerImpl.java:109) ~[purpur-1.21.4.jar:1.21.4-2412-9249a2c]
    at org.bukkit.plugin.SimplePluginManager.enablePlugin(SimplePluginManager.java:520) ~[purpur-api-1.21.4-R0.1-SNAPSHOT.jar:?]
    at org.bukkit.craftbukkit.CraftServer.enablePlugin(CraftServer.java:671) ~[purpur-1.21.4.jar:1.21.4-2412-9249a2c]
    at org.bukkit.craftbukkit.CraftServer.enablePlugins(CraftServer.java:620) ~[purpur-1.21.4.jar:1.21.4-2412-9249a2c]
    at net.minecraft.server.MinecraftServer.loadWorld0(MinecraftServer.java:746) ~[purpur-1.21.4.jar:1.21.4-2412-9249a2c]
    at net.minecraft.server.MinecraftServer.loadLevel(MinecraftServer.java:491) ~[purpur-1.21.4.jar:1.21.4-2412-9249a2c]
    at net.minecraft.server.dedicated.DedicatedServer.initServer(DedicatedServer.java:365) ~[purpur-1.21.4.jar:1.21.4-2412-9249a2c]
    at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:1178) ~[purpur-1.21.4.jar:1.21.4-2412-9249a2c]
    at net.minecraft.server.MinecraftServer.lambda$spin$2(MinecraftServer.java:313) ~[purpur-1.21.4.jar:1.21.4-2412-9249a2c]
    at java.base/java.lang.Thread.run(Thread.java:1583) ~[?:?]
[22:36:58] [Server thread/INFO]: [MythicMobs] Disabling MythicMobs v5.9.0-SNAPSHOT-d0a50305

Consistently replicated by installing latest dev builds on a fresh 1.21.4 Paper server. Installing 5.8.2 or dev builds prior to build 5388 provide the configuration needed for builds 5388+ to boot correctly.