Suddenly can't connect via Putty to OpenSSH server

From my Windows 7 machine, I've consistently been able to connect via SSH (using Putty) to two different Linux servers. A few weeks ago, one of them stopped responding to connection attempts. Putty would just sit there with a black screen and eventually time out. Troubleshooting attempts:

  • It wasn't a local outgoing firewall (or Windows 7) problem, because I could still connect to other OpenSSH servers
  • It wasn't a problem with the OpenSSH server itself, because I could SSH to one server, and then SSH from there to the 'problem' server. 

I finally found an entry like this in /var/log/auth.log:"error: Hm, kex protocol error: type 30 seq 1"A quick search turned up this post, which mentions that an upgrade to OpenSSH on the server can cause a common Key Exchange Algorithm (Kex), called the "Diffie-Hellman Group Exchange" to no longer work. It goes on to say, in Putty, you just select SSH->Kex, and then move the "Diffie-Hellman Group Exchange" to the bottom of the list to resolve the problem. 

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