A request to this url: "http://localhost/drupal/flex_image/folders" is being generated by the flex application. However, in my environment at least, the correct URL is: "http://localhost/drupal/index.php?q=flex_image/folders".

A workaround is to place the following directives in your apache configuration:
### SNIP ###
RewriteEngine on
RewriteRule ^/drupal/flex_image/(.*)$ /drupal/index.php?q=flex_image/$1 [R]
### SNIP ###

This redirects the URL and prevents the 404 error.