See my original reply above. The latest synology software has an updated ssh that would cause this problem. You may need to contact synology on how to change the key exchange algorithms if you can't just ssh into it and edit the file.
Not sure if you ever figured this out but I had the same problem after updating linux. OpenSSH removed a bunch of insecure ciphers which are still used by a good bit of Cisco stuff. With CCUM 10.0 I had to turn those ciphers back on. In my testing th...
