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I'm pretty new to Xoops, but have used PHP-Nuke for years. In the log in PHP-Nuke, 404 Page errors can be someone fishing for ways to hack, but quite unlikely. It's probably more to the fact that the person trying to login to your site using the cache was for some reason unable to load the page they previously were on.
Could be a number of reasons:
server popular and bandwidth could have been stretched or the user is using a slow machine and timed out before they were able to load the screen.
An idea to stop these from happening is to bounce users directly from a 404, 403 and even 500 Error Page straight to your Homepage.
This can be configured quite easily using CPanel. I believe .htaccess is able to do this as well.
Hope that helps