If you upgraded to macOS Sierra (beta or the new publicly available today 10/20/16), you might have issues with SSHing into servers – especially Linux. That’s because newer versions of Linux and Mac require better encryption.

The default has always been 3des, but you need to use at least aes128-ctr. Here’s an example of the old SSH command in Terminal:
ssh -c 3des -2 -l root -p 22 ;exit

Now, you’d need to update that to:

ssh -c aes128-ctr -2 -l root -p 22 ;exit