I have this problem for a few days. I try to comment on posts, but it gives me an error, which says that my comment is blocked. Moreover, I can't add captions under my posts. I don't comment abusive things, I don't use any apps to get more followers or likes, my posts, captions and comments are very casual. What's the problem?
I've upgraded Instagram and I tried to reinstall it, but nothing.
I also tried to log in using other device, but it still shows the same, so, I guess there's no problem with device.
I also tried to create new account, but Instagram asks to verify it by adding a number and after I do that, Instagram shows the same "comment block" thing, although it's completely new account.
Does anybody know a solution?

Try deleting the app and re-instaling it or try it on another device. You could also try logging in and out of your account.

Chances are you've violated their terms and conditions of use somehow. I found a great [Article] (http://www.quora.com/Why-wont-Instagram-let-me-add-comments-or-captions-to-my-photos) that might shed some like on this. All else fails, contact Instagram Tech support.