Error: Cound not schedule mailbox

Hey there, i’m currently running paper-146.jar. But i get an error when shutting down the server.

>.... [01:28:07 ERROR]: Cound not schedule mailbox
java.util.concurrent.RejectedExecutionException: null
at java.util.concurrent.ForkJoinPool.externalSubmit(ForkJoinPool.java:2328) ~[?:1.8.0_212]
at java.util.concurrent.ForkJoinPool.externalPush(ForkJoinPool.java:2419) ~[?:1.8.0_212]
at java.util.concurrent.ForkJoinPool.execute(ForkJoinPool.java:2648) ~[?:1.8.0_212]
at net.minecraft.server.v1_14_R1.ThreadedMailbox.f(SourceFile:112) ~[patched_1.14.4.jar:git- 
Paper-146]
at net.minecraft.server.v1_14_R1.ThreadedMailbox.a(SourceFile:101) ~[patched_1.14.4.jar:git- 
Paper-146]
at net.minecraft.server.v1_14_R1.ChunkTaskQueueSorter.a(SourceFile:142) ~ 
[patched_1.14.4.jar:git-Paper-146]
at net.minecraft.server.v1_14_R1.ChunkTaskQueueSorter.a(SourceFile:150) ~ 
[patched_1.14.4.jar:git-Paper-146]
at java.util.concurrent.CompletableFuture.uniAccept(CompletableFuture.java:656) ~[?:1.8.0_212]
at java.util.concurrent.CompletableFuture$UniAccept.tryFire(CompletableFuture.java:632) ~ 
[?:1.8.0_212]
at java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:474) ~ 
[?:1.8.0_212]
at java.util.concurrent.CompletableFuture.complete(CompletableFuture.java:1962) ~[?:1.8.0_212]
at net.minecraft.server.v1_14_R1.Mailbox$1.a(SourceFile:32) ~[patched_1.14.4.jar:git-Paper-146]
at net.minecraft.server.v1_14_R1.ChunkTaskQueueSorter.b(SourceFile:59) ~[patched_1.14.4.jar:git- 
Paper-146]
at net.minecraft.server.v1_14_R1.ThreadedMailbox.e(SourceFile:83) ~[patched_1.14.4.jar:git- 
Paper-146]
at net.minecraft.server.v1_14_R1.ThreadedMailbox.a(SourceFile:123) ~[patched_1.14.4.jar:git- 
Paper-146]
at net.minecraft.server.v1_14_R1.ThreadedMailbox.run(SourceFile:91) ~[patched_1.14.4.jar:git- 
Paper-146]
at java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1402) ~ 
[?:1.8.0_212]
at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289) ~[?:1.8.0_212]
at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056) ~[?:1.8.0_212]
at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692) ~[?:1.8.0_212]
at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157) ~[?:1.8.0_212]    

I’m only running 1 plugin called WorldBorder and it’s currently Pre-Generating chunks. I have no clue what this error means, tried to search for it but no luck. Anyone here can help me out?

Also Cound is supposed to be Could it’s a typo in Paper itself ~