Follow this template except for feature requests. Use pastebin when providing /protocol dump and any relevant errors.

Make sure you've done the following:

• You're using the latest build for your server version
• This isn't an issue caused by another plugin
• You've checked for duplicate issues
• You didn't use /reload

Console error when placing down a sign.
Error:
[14:35:20 FATAL]: Error executing task
java.util.concurrent.ExecutionException: java.lang.NullPointerException
at java.util.concurrent.FutureTask.report(FutureTask.java:122) ~[?:1.8.0_201]
at java.util.concurrent.FutureTask.get(FutureTask.java:192) ~[?:1.8.0_201]
at net.minecraft.server.v1_13_R2.SystemUtils.a(SourceFile:200) [spigot.jar:git-Spigot-f09662d-494eef4]
at net.minecraft.server.v1_13_R2.MinecraftServer.b(MinecraftServer.java:896) [spigot.jar:git-Spigot-f09662d-494eef4]
at net.minecraft.server.v1_13_R2.DedicatedServer.b(DedicatedServer.java:417) [spigot.jar:git-Spigot-f09662d-494eef4]
at net.minecraft.server.v1_13_R2.MinecraftServer.a(MinecraftServer.java:831) [spigot.jar:git-Spigot-f09662d-494eef4]
at net.minecraft.server.v1_13_R2.MinecraftServer.run(MinecraftServer.java:729) [spigot.jar:git-Spigot-f09662d-494eef4]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_201]
Caused by: java.lang.NullPointerException
at net.minecraft.server.v1_13_R2.EntityPlayer.openSign(EntityPlayer.java:794) ~[spigot.jar:git-Spigot-f09662d-494eef4]
at net.minecraft.server.v1_13_R2.ItemStack.placeItem(ItemStack.java:279) ~[spigot.jar:git-Spigot-f09662d-494eef4]
at net.minecraft.server.v1_13_R2.PlayerInteractManager.a(PlayerInteractManager.java:511) ~[spigot.jar:git-Spigot-f09662d-494eef4]
at net.minecraft.server.v1_13_R2.PlayerConnection.a(PlayerConnection.java:1221) ~[spigot.jar:git-Spigot-f09662d-494eef4]
at net.minecraft.server.v1_13_R2.PacketPlayInUseItem.a(PacketPlayInUseItem.java:37) ~[spigot.jar:git-Spigot-f09662d-494eef4]
at net.minecraft.server.v1_13_R2.PacketPlayInUseItem.a(PacketPlayInUseItem.java:1) ~[spigot.jar:git-Spigot-f09662d-494eef4]
at net.minecraft.server.v1_13_R2.PlayerConnectionUtils.lambda$0(PlayerConnectionUtils.java:9) ~[spigot.jar:git-Spigot-f09662d-494eef4]
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) ~[?:1.8.0_201]
at java.util.concurrent.FutureTask.run(FutureTask.java:266) ~[?:1.8.0_201]
at net.minecraft.server.v1_13_R2.SystemUtils.a(SourceFile:199) ~[spigot.jar:git-Spigot-f09662d-494eef4]
... 5 more

why would this be a protocollib issue? is it reproduceable without protocollib? what about protocollib but no other plugins?

It does only happen if protocollib is installed.

Run /protocol dump and paste the output with pastebin. Could be a misbehaving plugin that uses PL.

I can´t reproduce it anymore. Both time (even when the error occurred) there were no other plugins except ProtocolLib installed. I think i´ll close this now.