The error may look like this:


Couldn't agree a key exchange algorithm (available: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,)

The error may occur when connecting via SSH via clients such as Putty, Filezilla, WinSCP.

To solve the problem you need to update the client to the latest version via the built-in update function, or from the official website:

Putty: https://www.chiark.greenend.org.uk/~sgtatham/putty/latest.html
FileZilla: https://filezilla-project.org/download.php
WinSCP: https://winscp.net/eng/download.php