Every time I enter a minecraft server I get the message: java.lang.NullPointerException: group

Every time I enter a minecraft server I get the message: java.lang.NullPointerException: group, When I enter a player I do not load the world and close the minecraft, please help I'm very worried D: I'm premium, my name Is:, Help please, I Install and uninstall java and i erase the .minecraft and i download a new .minecraft, but gives me the same error! Help please!