sharepoint running on port 443 - error 404 not found

We know a few things. We know that a 404 is an HTTP error. We know that the BIG-IP does not send HTTP errors. And we know by virtue of the first thing that SSL isn't the issue. So there's a 99.999% chance that this is an application-level problem, and that the client is asking for something that server says doesn't exist. Applications can sometimes do some unusual things behind a proxy server. The best way to start troubleshooting this is with a client side browser-based capture like HTTPWatch or Fiddler. You could probably even get away with FireFox's Live HTTP Headers plugin. What you're looking for is the set of requests and responses leaving and returning to the browser, and which request triggers the 404.