This is a common problem after updating to 10.1. There's a Apple Knowledge Base article that will explain how to fix it in depth:
Unfortunately, you do need a username and a password to see this document. For a quicker explanation, check out this article:
It's under the "Apache breaks upon update" heading. I have to admit that I haven't run into this error myself, but the answers I've seen are solid and make sense, so things should go smoothly for you.