I have a similar problem.

I'm trying to connect to my ssh server on a raspberry pi with OpenSSH 7.5

Get this error in the Textastic Log:

== Info:   Trying 192.168.2.25...
== Info: TCP_NODELAY set
== Info: Connected to 192.168.2.25 (192.168.2.25) port 22 (#0)
== Info: Failure establishing ssh session
== Info: Closing connection 0

And this on the server side:

Connection from 192.168.2.15 port 49722 on 192.168.2.25 port 22
Unable to negotiate with 192.168.2.15 port 49722: no matching key exchange method found.
Their offer: diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1 [preauth]

Connection from other clients works fine.

I can set a "compatibility_mode" on my server and than it works, but its less secure.