I have been using the Flexible Upload D3Z Edition WordPress plugin for a couple years now on various sites that I work on or manage. To me it provides a much easier interface to upload images while building thumbnails on the fly than the default WordPress image media upload. The issue is the Flexible Upload D3Z Edition plugin has not been updated since WordPress version 2.6.X so it has broken on more than one occasion. So when upgrading to WordPress 3 recently it broke the Flexible Upload D3Z Edition plugin and below I describe how to fix the plugin so it will operate without issue in WordPress 3.X.
I have been having a problem with the media browser uploader for a couple days since I enabled FORCE_SSL_ADMIN in my wp-config.php file. I finally got some time tonight to look into the issue and I am embarrassed it took me so long to resolve. This was a classic example of trying to make something more complicated than it really was. The issue ended up being with a WordPress plugin I had installed called Flexible Uploader. This plugin provides all the functionality of the flash media uploader to the browser media uploader. It turns out that this plugin has not been updated in a long time and some of the URL building functions have changed in the newer versions of WordPress. Below I explain how to easily resolve the issue.