I upgraded to OpenSSH_6.9p1 Ubuntu-2ubuntu0.1 and can no longer connect via Core FTP Pro to the server.
This is the output from Core FTP:
Server version: SSH-2.0-OpenSSH_6.9p1 Ubuntu-2
Using SSH protocol version 2
Version: SSH-2.0-PuTTY_Local:_Dec_11_2015_18:20:00
Diffie-Hellman group exchange
Server unexpectedly closed network connection
Can't establish connection --> ***.***.***.***:*** @ Thu Jan 14 12:01:11 2016 (10038-1)
This is what is in auth.log on the server:
Jan 14 11:54:26 DevServer sshd[3192]: Connection closed by ***.***.***.*** [preauth]
Jan 14 11:59:26 DevServer sshd[3196]: error: Hm, kex protocol error: type 30 seq 1 [preauth]
I am able to connect via Putty, SQLYog, and Filezilla but not Core.