Kex error: Get x2goclient to work on Raspberry pi

The Raspbian version of X2GoClient is too old to connect to an X2GoServer for Ubuntu 15.04:

wiki:repositories:start [X2Go - everywhere@home] - Newer / more packages are not available currently. X2Go's upstream package archive does not include Raspbian packages.

You can get the Raspbian x2goclient to work again. You need to configure the ssh server (Ubuntu 15.04 in my case) where the x2goserver resides to accept deprecated (and hence somewhat insecure) ciphers. This line added to my sshd_config made it possible:

KexAlgorithms curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha1,diffie-hellman-group-exchange-sha1,diffie-hellman-group1-sha1

Do note that it should all be on one line, especially the "KexAlgorithms" should not be on its own line. Because then ssh logins get disabled wholesale, I am privy to know.

And you will need to delete the server from the ~/.ssh/known_hosts file, or x2goclient will refuse to connect.