So you reached that hidden Twitter error after 2000 perfectly okay follows.
Here’s the problem: You can’t follow more than 2000 users on Twitter until you have over 1800 followers. Afterwards, you can follow 10% more users than follow you.
If you hit the 2,000 limit, unfollow some and then you can follow more. Just unfollow. It’s okay, sometimes things don’t work out.
ManageFlitter.com is a great tool for unfollowing folks that never followed you back after a while. It has a bunch of nice sorting features. Other folks like JustUnfollow.com for something more automated.