When anyone other than me (admin) tries to change their password they get the following error:
Sorry, you need to enable sending referrers for this feature to work.
I have checked their options within their computer, but still got the problem. What could be causing this problem?
Probably a bug in the referrer checking code with it not liking URL's other than that of the main blog, although that is a total guess so I would grep the source for that error message and see what it is looking at in order to throw that error.
Where actually is the source code? I can only find the nightly builds.
The nightly builds are your source... decompress them into a folder and start your search from there... Or if you have it installed and you don't mind messing with live code, go ahead and look for it in the directory that you have the software installed in. Hope that helps you a bit.