We have gotten this on a few RB3011s.
This has been after they were 'comprimised', and then netinstalled. Still unable to access them. We have not yet found a solution.
That did the trick. Unfortunately I have one board that has attempts via FTP. I've changed the ftp port as well and the attempts still occur. Any suggestions?Also - if you really want to make it better then just change your ssh port from 22 to something else.