Can't join muliplayer game/server - java.lang.NullPointerException

when i click on server from client get the following message:

Failed to connect to the server

Internal Exception: java.lang.NullPointerException

On server get the following message:

[10:30:24] [Server thread/INFO]: com.mojang.authlib.GameProfile@1c426f7[id=<null>,name=Unknown,properties={},legacy=false] (/192.168.0.20:51808) lost connection: Disconnected