Just like #11 this is because AnvilGUI has not been updated.
[14:04:36 ERROR]: Could not pass event InventoryClickEvent to HeadDB v2.3.1
java.lang.NoClassDefFoundError: Could not initialize class net.wesjd.anvilgui.AnvilGUI
at net.wesjd.anvilgui.AnvilGUI$Builder.open(AnvilGUI.java:491) ~[?:?]
at tsp.headdb.inventory.PagedPane.lambda$createControls$4(PagedPane.java:277) ~[?:?]
at tsp.headdb.inventory.Button.onClick(Button.java:61) ~[?:?]
at tsp.headdb.inventory.PagedPane.onClick(PagedPane.java:179) ~[?:?]
at tsp.headdb.listener.PagedPaneListener.onClick(PagedPaneListener.java:24) ~[?:?]
at com.destroystokyo.paper.event.executor.asm.generated.GeneratedEventExecutor159.execute(Unknown Source) ~[?:?]
at org.bukkit.plugin.EventExecutor.lambda$create$1(EventExecutor.java:69) ~[patched_1.17.jar:git-Paper-71]
at co.aikar.timings.TimedEventExecutor.execute(TimedEventExecutor.java:80) ~[patched_1.17.jar:git-Paper-71]
at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:70) ~[patched_1.17.jar:git-Paper-71]
at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:624) ~[patched_1.17.jar:git-Paper-71]
at net.minecraft.server.network.ServerGamePacketListenerImpl.handleContainerClick(ServerGamePacketListenerImpl.java:2728) ~[?:?]
at net.minecraft.network.protocol.game.ServerboundContainerClickPacket.handle(ServerboundContainerClickPacket.java:50) ~[?:?]
at net.minecraft.network.protocol.game.ServerboundContainerClickPacket.handle(ServerboundContainerClickPacket.java:11) ~[?:?]
at net.minecraft.network.protocol.PacketUtils.lambda$ensureRunningOnSameThread$1(PacketUtils.java:36) ~[?:?]
at net.minecraft.server.TickTask.run(TickTask.java:18) ~[patched_1.17.jar:git-Paper-71]
at net.minecraft.util.thread.BlockableEventLoop.doRunTask(BlockableEventLoop.java:149) ~[?:?]
at net.minecraft.util.thread.ReentrantBlockableEventLoop.doRunTask(ReentrantBlockableEventLoop.java:23) ~[?:?]
at net.minecraft.server.MinecraftServer.doRunTask(MinecraftServer.java:1340) ~[patched_1.17.jar:git-Paper-71]
at net.minecraft.server.MinecraftServer.shouldRun(MinecraftServer.java:193) ~[patched_1.17.jar:git-Paper-71]
at net.minecraft.util.thread.BlockableEventLoop.pollTask(BlockableEventLoop.java:122) ~[?:?]
at net.minecraft.server.MinecraftServer.pollTaskInternal(MinecraftServer.java:1319) ~[patched_1.17.jar:git-Paper-71]
at net.minecraft.server.MinecraftServer.pollTask(MinecraftServer.java:1312) ~[patched_1.17.jar:git-Paper-71]
at net.minecraft.util.thread.BlockableEventLoop.managedBlock(BlockableEventLoop.java:132) ~[?:?]
at net.minecraft.server.MinecraftServer.waitUntilNextTick(MinecraftServer.java:1273) ~[patched_1.17.jar:git-Paper-71]
at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:1184) ~[patched_1.17.jar:git-Paper-71]
at net.minecraft.server.MinecraftServer.lambda$spin$0(MinecraftServer.java:320) ~[patched_1.17.jar:git-Paper-71]
at java.lang.Thread.run(Thread.java:831) [?:?]
Just like #11 this is because AnvilGUI has not been updated.