Wordpress Problem - Subcategory Listings Give Error

Hey,
I'm using permalinks to change the general link structure of my Wordpress blog, but it hasn't done so well with my subcategories, regular categories - fine, subcategories - lead to an error.

My website is:
http://www.dealoverflow.com/

When you go to a subcategory, http://www.dealoverflow.com/computers/laptop/, the website leads to an error.

A regular category, http://www.dealoverflow.com/computers/, is fine, though.

 

Can you show us your .htaccess?

 

Here's my .hataccess:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

#BEGIN Image Upload HTTP Error Fix
<IfModule mod_security.c>
<Files async-upload.php>
#SecFilterEngine Off
#SecFilterScanPOST Off
</Files>
</IfModule>
<IfModule security_module>
<Files async-upload.php>
#SecFilterEngine Off
#SecFilterScanPOST Off
</Files>
</IfModule>
<IfModule security2_module>
<Files async-upload.php>
#SecFilterEngine Off
#SecFilterScanPOST Off
</Files>
</IfModule>
#END Image Upload HTTP Error Fix

# END WordPress

Code (markup):