I have the same problem with the "no tweets available" message, though the widget itself does show up with the twitter username.
I'm running version 2.4.1 and I made sure that 'http' is checked in the settings instead of 'https.' I cleared update locks, but "There were no locks to clear." Also, when the API Usage Limits reset, it still doesn't work. I even added '@' in front to the username, but it still isn't working.