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1. User logs in (Quick Search is not available to non-members and I confirmed that whether a regular user or administrator the steps below fail).
2. Once you're logged in, the system shows the Quick Search dialog box on the right side of the page (see attached).
3. User doesn't enter anything, just clicks the link for "Advanced Search"
4. System presents the 404 - Not Found error (see attached).
Via Google, I found another reference on your site that fixed this problem.
Following that previous post, I searched for and changed this line in the en.GB.com_community.ini file, removing both _QQQ_ references and the search now works.