It appears that OpalStack enforces a limit of the number of simultaneous connections over SSH. I cannot locate anywhere that limit is published, but I constantly run up against that limit when attempting to upload a number of files over SFTP. Once the limit is reached, I must wait an hour for the lockdown to timeout before I can continue updating my application.
After searching through the forums, I can see that I am not the only one running into these issues. My wish is that OpalStack will either publish their connection limits OR modify them altogether so as to eliminate the issue.
P.S. this exact same problem occurred on WebFaction. Considering OpalStack derived from WebFaction, I am assuming that the issue is the same. On WebFaction and through trial and error, I had to limit my SFTP client to 5 simultaneous connections to work around this issue.