If you are getting this error, use this fix but make sure to shut off any WP applications that may be causing it! Not to go into specific details cause it is a pain in the neck but certain applications that can-rewrite urls for different browsing experiences are really messing up within WP. Selectively de-activate apps and wait to see if it works.
I cannot remember the name of the app that was doing it or I would tell you.
This is a quick fix i found that worked from yongee.hubpages.com and just wanted to make sure that the information gets spread around and to acknowledge that it does work.
Modify your .htaccess file, or rename it .htaccessOLD and start from scratch with a new .htaccess file.
# BEGIN WordPress
ErrorDocument 404 /index.php?error=404
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [fusion_builder_container hundred_percent="yes" overflow="visible"][fusion_builder_row][fusion_builder_column type="1_1" background_position="left top" background_color="" border_size="" border_color="" border_style="solid" spacing="yes" background_image="" background_repeat="no-repeat" padding="" margin_top="0px" margin_bottom="0px" class="" id="" animation_type="" animation_speed="0.3" animation_direction="left" hide_on_mobile="no" center_content="no" min_height="none"][L]
# END
## This kept giving me 404 ERRORS with WORDPRESS and HOSTGATOR GRRRRRRR ###
##WordPressRewriteEngine On
##RewriteCond %{HTTP:X-WAP-PROFILE} !^$ [OR]
##RewriteCond %{HTTP_ACCEPT} application/vnd.wap.xhtml\+xml [NC,OR]
##RewriteCond %{HTTP_ACCEPT} text/vnd.wap.wml [NC]
This has only happened for me with Host Gator. Nothing wrong with HG, they have the best support I have worked with.[/fusion_builder_column][/fusion_builder_row][/fusion_builder_container]