10.5 ‘The first cipher supported by the server is ... below the configured warning threshold’

This occurs when the SSH server does not offer any ciphers which you have configured PuTTY to consider strong enough. By default, PuTTY puts up this warning only for Blowfish, single-DES, and Arcfour encryption.

See section 4.20 for more information on this message.

(There are similar messages for other cryptographic primitives, such as host key algorithms.)