In this case the cause was that on the target system changes had been made to the sshd_config file to restrict certain MACs. For example:
This config is commenting out all MACs except for hmac-sha2-512,hmac-sha2-256,hmac-ripemd160
The MCU is looking for hmac-md5 or hmac-sha1
The discrepancy can be seen in the MCU debug log files:
This is the line from the debug stderrout showing the issue:
** 13:03:01.307 SSH2Transport.disconnectInternal() : 'disconnect: No match in kex params 'mac-algorithms-cli2srv', our's: hmac-md5,hmac-sha1, peer's: hmac-sha2-512,hmac-sha2-256,hmac-ripemd160'
This is an MCU issue and Product Defect #753453 has been raised to address the issue in a future release.
In MCU 2.5.1 the only fix is to enable one of the MACs that MCU is looking for either hmac-md5 or hmac-sha1 in sshd_config.
* Update with regards to product defect 753453, code was added in MCU 220.127.116.1114 so that the MCU now supports hmac-sha2-256.