Withdrawing a paper after acceptance due to error I had a paper accepted at a conference in theoretical computer science. However while revising the final version I found an error in the proof of an important lemma. Although I believe the error is fixable, It will require some effort. I will not be able to fix the error before the deadline for the final version. 

1) Should I withdraw the paper?

2) Should I leave the lemma without proof, and add the corrected proof at the final version.

What would be the best thing to do? Would I be able to resubmit the paper to some other conference? What would be the damage for my reputation?
I would contact the programme chair of the conference and ask for advice (perhaps suggest the options you have already given).  It could be that by submitting the paper you are under an obligation to attend, in which case you should seek permission from the conference chair to withdraw the paper.  To protect your reputation, at the very least explicitly state in the draft that lemma is currently unproven.   We all make mistakes, without exception, so we should all be understanding when they happen to others.