No entering a plot world on a server due to Java errors and Minecraft crash

Hello,

if I enter various City Builds servers or servers with a Plotworld I get a message from Jawa in the game outpout on my old laptop. Minecraft crashes and the Launcher starts again where there is a message that there was a mistake and Mojang apologizes for it. But I do not know if it would be synonymous, because I have today changed my Minecraft name of Jomo2003 in Jomo_HD.
In any case, here is the error message from Jawa from the Game Outpout:

18:37:24 ave client thread fatal error executing task
java.util.concurrent.ExecutionException: java.lang.NullPointerException
at java.util.concurrent.FutureTask.report (FutureTask.java:122)
at java.util.concurrent.FutureTask.get (FutureTask.java:192)
at g.a (SourceFile: 45)
at ave.av (SourceFile: 881)
at ave.a (SourceFile: 325)
at net.minecraft.client.main.Main.main (SourceFile: 124)
Caused by: java.lang.NullPointerException
at auo.k (SourceFile: 179)
at bcy.a (SourceFile: 1313)
at hq.a (SourceFile: 54)
at hq.a (SourceFile: 10)
at fh $ 1.run (SourceFile: 13)
at java.util.concurrent.Executors $ RunnableAdapter.call (Executors.java:511)
at java.util.concurrent.FutureTask.run (FutureTask.java:266)
at g.a (SourceFile: 44)
... 3 more

If you know what I can do about it, write me a comment.

The Jawa PC version is 1.8.9

LG and thanks in advance