How to answer a non-specific point of a review? I'm in the rebuttal stage of a conference paper and I'm having trouble to answer in a specific way to a point raised by a reviewer. The reason is that is completely non-specific:


  [...] and some subtle conditions are not discussed by the authors.


and the previous sentences are unrelated with the final one, so I don't have a clue on what he's speaking about.

It's quite frustrating, since it seems to me very unfair that he/she doesn't give me the opportunity to defend my paper. How can I answer in some constructive way?
I guess I won't be too popular for saying this, but you seem to have the wrong idea about what is going on. The point isn't to "defend your paper" or "rebut the reviewer" but to take constructive criticism of the paper so that you can improve it for the conference and for the future. It isn't a debate. It is a search for a way to express some scientific truths through a cooperative process. 

That said, you can't respond to what you don't know. As user Miguel says in a comment, the editor may be able to help you here if you say that you don't understand the comment and ask if the reviewer can give more specific advice. 

When you get the advice, hopefully you will, you can take it into account for a revision. But not just to complain about it. You won't win much by arguing. Your paper will only be accepted if it stands on its own, without side arguments in its favor. 

Please note that I'm not being insulting here. Nor were your reviewers if they made suggestions about the paper that you didn't really want to read.