Welcome to the Centrify Express forums.
By default, modern versions of Ubuntu with OpenSSH 7.x won't allow older clients (like the PuTTY client included with 5.3.1) to do the key exchange.
From: http://www.openssh.com/txt/release-7.0
We plan on retiring more legacy cryptography in the next release including: * Refusing all RSA keys smaller than 1024 bits (the current minimum is 768 bits) * Several ciphers will be disabled by default: blowfish-cbc, cast128-cbc, all arcfour variants and the rijndael-cbc aliases for AES. * MD5-based HMAC algorithms will be disabled by default.
The current supported version of Centrify Express is 5.3.1. Please upgrade your DirectManage components and repeat your testing.
Please