In order to keep their servers running smoothly, Twitter has established a set of very reasonable limits on the number of times users can do things when using an external app like Happy Friends.
I've tried to tune the software up in such a way that it's not likely to happen, but it's possible that with a large number of friends, or if you're checking for updates frequently, that you'll hit a rate-limit error. You'll know you're getting these errors if a red message appears below the outline that tells you. It links to this page.
Consider turning off automatic checks, or checking less frequently. In the menu at the right edge of the menu bar, choose the Settings command and click on the Rate panel.
If you turn off automatic checks, you can do a scan manually with the Check For Updates Now command in the Friends menu.
Watch fewer people in Happy Friends and keep the rate of automatic checks as-is.
Slow down, or take a break. Sometimes rate-limit errors can happen if you do too much expanding. So slow down a little, or do something else for a while. The rate-limits eventually reset (that is, Twitter forgets about the past excess).
When you come back, reload the page if the message is still there.
Here's what Twitter says to developers like me about rate limits. It's pretty deep stuff.