You guys (starting with Tony) are totally right. It was a security permissions thing. I'd set the sharing permissions but not the security ones, which is annoying.

But the major confusion was this: when I set the VPN up initially, I had about four computers connecting to the VPN and the server, and seeing all the files I wanted them to. Between then and today, I had changed absolutely nothing, but now those computers couldn't see those files. It's like something in the computer realized "oh, hey! I forgot that I need to apply the security permissions too! Well lets stop this now, and give off an error message with little to no meaning."

It appears to be working now, but naturally I have no faith in Windows' file sharing. I never did before, though, and this experience hasn't helped.