TOPIC: The word 'makeup' has become an invalid category slug

Hello. This is regarding my wife's makeup blog, www.makeupandbeautyblog.com. We were running WP 2.1 until last Friday when I upgraded to 2.5 to fix an issue it was having with all of the posts of a specific category (the makeup category), but the upgrade (while it went well) didn't alter the bizarre behavior.

Suddenly, about a week ago, all of the posts in which the first category (the one whose category slug WP uses in the URL at /%category%/) had been the word 'makeup' started returning Page Not Found errors... but if I went up to the browser address bar and alter the URL to anything other than the specific category slug word, it then found and displayed the post correctly. It's as if Wordpress was saying that 'makeup' was no longer a valid category slug, even though it had worked perfectly for more than a year.

This affected 83 of my posts, breaking all of the links to them on Google. I've tried renaming the category, then naming it back to the now forbidden word makeup -- no luck. Also tried the same with the category slug itself to no avail.

What it boils down to is a certain word, in this case makeup just no longer seems to be valid as a category slug name. It's still valid as the category name but not as the slug name, which is the word used to build the URL.

I'd like to ask if anyone here has any ideas what might have caused this. I've logged into the CPanel on my host (godaddy) and tried manually retyping the category and slug in the category table, but that didn't change the behavior. Also tried repairing and optimizing the table, but no change in the behavior either. None of my categories, user names, category slugs or posts are named the single word 'makeup' so as far as I can tell it is not a duplicate content issue.

When I rename the category, the newly created (changed) URL works correctly, but when I rename it back to the forbidden word (makeup), I get a Page Not Found error again.

This URL is valid...
http://www.makeupandbeautyblog.com/c...-07-spotlight/

But this URL (which had worked up until about a week ago) isn't...
http://www.makeupandbeautyblog.com/m...-07-spotlight/

And, perhaps because of the way mod_rewrite rewrites URLs, this URL (or any URL in which the /%category%/ section includes something other than 'makeup' or 'category') is valid...
http://www.makeupandbeautyblog.com/m...-07-spotlight/

Thanks for any and all input.

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UPDATE: I put into place an unpleasant workaround using the .htaccess file and a permanent redirect to point all incoming links destined for /makeup/post-title to /cosmetics/post-title... since it doesn't matter to wordpress what word gets used in the URL for the category (%category%) spot, just as long as (in this bizarre case) it isn't the word makeup.

As far as SEO is concerned, the word 'makeup' is preferable to the word 'cosmetics' so I would love to be able to figure this out. I've been asking for help from different communities (including wordpress.org's support forum) for a full week and have yet to receive any input, so I'm putting this call out there again in the hopes that someone tries to help. Thanks very much in advance.

I would really appreciate any help unraveling this mystery.

Check your actual wwwroot folder to see if somehow a folder called makeup has been created in it as this would give a 404.

Let me know if this works, otherwise we'll look at other scenarios