Error authenticating via SSH

One of our clients is attempting to start an SFTP session on our file transfer server. When they try to authenticate we see these errors in the auth.log.

Jan 16 11:24:34 ft sshd[6898]: error: kex protocol error: type 30 seq 1 [preauth]
Jan 16 11:24:34 ft sshd[6898]: error: Received disconnect from xxx.xxx.xxx.xxx port xxxxx:3: Expected SSH_MSG_KEX_GEX_GROUP [preauth]
Jan 16 11:24:34 ft sshd[6898]: Disconnected from xxx.xxx.xxx.xxx port xxxxx [preauth]

We can’t figure out why this client cannot connect and all other clients can connect.

Versions: Ubuntu 16.04.1 LTS OpenSSH_7.2p2 Ubuntu-4ubuntu2.1 OpenSSL 1.0.2g

Edit: Forgot to mention the fact that our client can connect using their local machines, the issue is that their production server cannot connect. We have set up our sshd_config to permit all available KexAlgorithms and Ciphers as listed by man sshd_config.

I’m just trying to understand what exactly is failing. Is it their server failing to establish an SSH connection to begin the authentication process?

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