Cannot connect: "unexpected message type 30 (expected one of [34])" reason found in log file

Some clients cannot connect to your Syncplify Server! version 6.x, and when you analyze the worker process (the virtual site's) log file you see the following message in it:

ssh: unexpected message type 30 (expected one of [34])

Why does this happen?

The message "type 30" is a pre-RFC4419 group exchange message, and support for the "type 30" message has been removed from OpenSSH release 6.1 and later. To prevent potential vulnerabilities and in an always-ongoing effort to keep our custoemrs and users as safe as possible, we removed it too from our Syncplify Server! software. When this default message in the Diffie-Hellman group exchange algorithm policy is used during SSH key exchange, it generates and logs the above error message, and the key-exchange attempt is aborted by the server.

The solution is to upgrade or reconfigure the client software so that it sends the new and correct message type 34 to request the usage of any diffie-hellman-group-exchange key exchange algorithm.