Tiny MCE Image Editor showing "No Image Available"

I have a MODx Evo 1.0 site with Tiny MCE that is showing "No Images Available" in the image editor (see attachment).

I read the documentation at the bottom of this page: http://wiki.modxcms.com/index.php/TinyMCE ... has anyone else run into this problem and found a remedy? For the File Browser URL in the configuration settings, I have "/assets/".