The problem may be related to the potentially incompatible changes introduced in OpenSSH 6.7 (included in Copssh 5 and higher) to remove unsafe algorithms.

• If you run Copssh 5.8.1 or higher, you can update the configuration via GUI - Configuration -> Advanced -> KexAlgorithms, set value to +diffie-hellman-group1-sha1
• Alternatively, you can add following line to the section of the configuration file control/bin/copsshcp.config before starting Copssh Control Panel:

KexAlgorithms=+diffie-hellman-group1-sha1

• Restart the service via Copssh Control Panel