Pretty sure
this guy's problem is related to Amber Client, as Loftar said it was a custom client issue and I'm currently experiencing the same thing with Amber Client, but not Ender's antiquated one.
java.lang.IllegalArgumentException: hostname can't be null
at java.net.InetSocketAddress.checkHost(Unknown Source)
at java.net.InetSocketAddress.<init>(Unknown Source)
at haven.SslHelper.connect(SslHelper.java:155)
at haven.AuthClient.<init>(AuthClient.java:49)
at haven.Bootstrap.run(Bootstrap.java:149)
at haven.MainFrame.run(MainFrame.java:268)
at java.lang.Thread.run(Unknown Source)