This error showed up when upgrading to 2.4 (1.16.5)
[13:14:44 INFO]: UUID of player Danik is 8b7bb4b2-a1ed-3244-b55c-95697249ee07 [13:14:45 ERROR]: Could not pass event PlayerJoinEvent to Minepacks v2.3.24-Release java.lang.NoClassDefFoundError: at/pcgamingfreaks/MinepacksStandalone/Bukkit/Database/Database$2 at at.pcgamingfreaks.MinepacksStandalone.Bukkit.Database.Database.asyncLoadBackpack(Database.java:253) ~[?:?] at at.pcgamingfreaks.MinepacksStandalone.Bukkit.Database.Database.updatePlayerAndLoadBackpack(Database.java:280) ~[?:?] at at.pcgamingfreaks.MinepacksStandalone.Bukkit.Database.Database.onPlayerLoginEvent(Database.java:273) ~[?:?] at com.destroystokyo.paper.event.executor.asm.generated.GeneratedEventExecutor50.execute(Unknown Source) ~[?:?] at org.bukkit.plugin.EventExecutor.lambda$create$1(EventExecutor.java:69) ~[patched_1.16.5.jar:git-Paper-777] at co.aikar.timings.TimedEventExecutor.execute(TimedEventExecutor.java:80) ~[patched_1.16.5.jar:git-Paper-777] at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:70) ~[patched_1.16.5.jar:git-Paper-777] at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:624) ~[patched_1.16.5.jar:git-Paper-777] at net.minecraft.server.v1_16_R3.PlayerList.postChunkLoadJoin(PlayerList.java:356) ~[patched_1.16.5.jar:git-Paper-777] at net.minecraft.server.v1_16_R3.PlayerList.lambda$a$1(PlayerList.java:303) ~[patched_1.16.5.jar:git-Paper-777] at net.minecraft.server.v1_16_R3.PlayerConnection.tick(PlayerConnection.java:316) ~[patched_1.16.5.jar:git-Paper-777] at net.minecraft.server.v1_16_R3.NetworkManager.a(NetworkManager.java:408) ~[patched_1.16.5.jar:git-Paper-777] at net.minecraft.server.v1_16_R3.ServerConnection.c(ServerConnection.java:168) ~[patched_1.16.5.jar:git-Paper-777] at net.minecraft.server.v1_16_R3.MinecraftServer.b(MinecraftServer.java:1515) ~[patched_1.16.5.jar:git-Paper-777] at net.minecraft.server.v1_16_R3.DedicatedServer.b(DedicatedServer.java:436) ~[patched_1.16.5.jar:git-Paper-777] at net.minecraft.server.v1_16_R3.MinecraftServer.a(MinecraftServer.java:1342) ~[patched_1.16.5.jar:git-Paper-777] at net.minecraft.server.v1_16_R3.MinecraftServer.w(MinecraftServer.java:1130) ~[patched_1.16.5.jar:git-Paper-777] at net.minecraft.server.v1_16_R3.MinecraftServer.lambda$a$0(MinecraftServer.java:291) ~[patched_1.16.5.jar:git-Paper-777] at java.lang.Thread.run(Thread.java:831) [?:?] Caused by: java.lang.ClassNotFoundException: at.pcgamingfreaks.MinepacksStandalone.Bukkit.Database.Database$2 at org.bukkit.plugin.java.PluginClassLoader.loadClass0(PluginClassLoader.java:155) ~[patched_1.16.5.jar:git-Paper-777] at org.bukkit.plugin.java.PluginClassLoader.loadClass(PluginClassLoader.java:114) ~[patched_1.16.5.jar:git-Paper-777] at java.lang.ClassLoader.loadClass(ClassLoader.java:519) ~[?:?] ... 19 more [13:14:45 INFO]: Danik[/109.81.210.170:57372] logged in with entity id 61 at ([Survival]1581.7302448389353, 80.0, -3806.6410857266474) [13:14:45 WARN]: [Minepacks] Plugin Minepacks v2.3.24-Release generated an exception while executing task 10647 java.lang.NoClassDefFoundError: at/pcgamingfreaks/MinepacksStandalone/libs/com/zaxxer/hikari/pool/HikariProxyPreparedStatement at at.pcgamingfreaks.MinepacksStandalone.libs.com.zaxxer.hikari.pool.ProxyFactory.getProxyPreparedStatement(ProxyFactory.java) ~[?:?] at at.pcgamingfreaks.MinepacksStandalone.libs.com.zaxxer.hikari.pool.ProxyConnection.prepareStatement(ProxyConnection.java:315) ~[?:?] at at.pcgamingfreaks.MinepacksStandalone.libs.com.zaxxer.hikari.pool.HikariProxyConnection.prepareStatement(HikariProxyConnection.java) ~[?:?] at at.pcgamingfreaks.MinepacksStandalone.Bukkit.Database.SQL.runStatement(SQL.java:248) ~[?:?] at at.pcgamingfreaks.MinepacksStandalone.Bukkit.Database.SQL.lambda$runStatementAsync$0(SQL.java:243) ~[?:?] at org.bukkit.craftbukkit.v1_16_R3.scheduler.CraftTask.run(CraftTask.java:100) ~[patched_1.16.5.jar:git-Paper-777] at org.bukkit.craftbukkit.v1_16_R3.scheduler.CraftAsyncTask.run(CraftAsyncTask.java:54) ~[patched_1.16.5.jar:git-Paper-777] at com.destroystokyo.paper.ServerSchedulerReportingWrapper.run(ServerSchedulerReportingWrapper.java:22) ~[patched_1.16.5.jar:git-Paper-777] at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1130) ~[?:?] at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:630) ~[?:?] at java.lang.Thread.run(Thread.java:831) [?:?] Caused by: java.lang.ClassNotFoundException: at.pcgamingfreaks.MinepacksStandalone.libs.com.zaxxer.hikari.pool.HikariProxyPreparedStatement at org.bukkit.plugin.java.PluginClassLoader.loadClass0(PluginClassLoader.java:155) ~[patched_1.16.5.jar:git-Paper-777] at org.bukkit.plugin.java.PluginClassLoader.loadClass(PluginClassLoader.java:114) ~[patched_1.16.5.jar:git-Paper-777] at java.lang.ClassLoader.loadClass(ClassLoader.java:519) ~[?:?] ... 11 more

This looks to me like your download is defective. Please try to download it again directly form spigot or github.

Ohh, I actually haven't installed the newest for some reason, I must have mistakly not put it there or idk. I am very sorry, my bad.