Attempting to debug a problem like this with only the limited information supplied is nearly, if not completely, impossible. However, there are a couple of things to look for, all of which involve permission settings. First, I am assuming that one of the two users is "root", and that is the one for which everything works. If that is the case, some file(s) and/or directory(ies) don't have the correct permissions for "world" or "other." Unfortunately, without getting to your computer to try to debug, it is impossible for me to tell you even where to look, as I have no idea where you tried to install the X windows software. If both of the users you speak of are not root, then likely you installed X windows as one of those two users, and that is the user which can access. Again, the problem is probably with the permissions and without looking at your machine, I can't tell you what you did wrong. I'm afraid you'll have to seek help from someone that can actually access your machine to do debugging.
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