It happens primary if the apache virtual hosts handler or the .htaccess file handler recognizes a pattern in the request and forwards a different request. In other words there is most likely a misconfiguration in the .htaccess file.
You should also check if the file is accessible directly from the browser.
Other causes might be bad permissions or ownership settings, but I don't think this is the case.
Also, make sure that you don't have anything in your system “hosts” file ralted to that domain.Best,
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