A member of my server was disconnected and this error log was generated, he is no longer able to connect and for a while (until a server restart) He still showed online in the world. 

---- Minecraft Crash Report -------- Minecraft Crash Report ----// I bet Cylons wouldn't have this problem.
Time: 2/24/19 3:33 PMDescription: Exception in server tick loop
java.util.concurrent.CompletionException: j: Writing into PalettedContainer from multiple threads at java.util.concurrent.CompletableFuture.encodeThrowable(Unknown Source) at java.util.concurrent.CompletableFuture.completeThrowable(Unknown Source) at java.util.concurrent.CompletableFuture.uniCompose(Unknown Source) at java.util.concurrent.CompletableFuture$UniCompose.tryFire(Unknown Source) at java.util.concurrent.CompletableFuture$Completion.run(Unknown Source) at uk.b(SourceFile:57) at afr.f(SourceFile:83) at afr.a(SourceFile:123) at afr.run(SourceFile:91) at java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(Unknown Source) at java.util.concurrent.ForkJoinTask.doExec(Unknown Source) at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(Unknown Source) at java.util.concurrent.ForkJoinPool.runWorker(Unknown Source) at java.util.concurrent.ForkJoinWorkerThread.run(Unknown Source)Caused by: j: Writing into PalettedContainer from multiple threads at ui.a(SourceFile:417) at com.mojang.datafixers.util.Either$Left.map(Either.java:38) at ui.a(SourceFile:400) ... 12 moreCaused by: java.lang.IllegalStateException at btl.a(SourceFile:42) at btl.a(SourceFile:109) at btf.a(SourceFile:60) at btf.a(SourceFile:54) at btm.a(SourceFile:178) at cfe.a(SourceFile:74) at cfe.a(SourceFile:21) at cfe.a(SourceFile:14) at cfd.a(SourceFile:35) at bec.a(SourceFile:407) at bun.c(SourceFile:238) at bsx.f(SourceFile:54) at bsx$c.doWork(SourceFile:220) at bsx.a(SourceFile:184) at ui.a(SourceFile:406) ... 14 more