Thank you for making the effort to load and test the Development Version, and for the additional information about the problem, which is very helpful.
When you say “no images show up“, is it only the image thumbnails that fail to appear? Do you see the toolbar at the top of the window with the dropdown controls and the search box?
Your issue may be similar to this one from an earlier topic:
MLA breaking Add Featured Image
In that case a combination of plugins caused the problem. The problem only occurred when the WordPress “CONCATENATE_SCRIPTS” option was set to “true” (the default). You could try adding a line to your wp-config.php file to avoid the conflict: define(‘CONCATENATE_SCRIPTS’, false); This line must be above the ” require_once(ABSPATH . ‘wp-settings.php’); line in the wp-config.php file.
You can also try disabling the MLA enhancements instead of deactivating MLA completely. Go to the Settings/Media Library Assistant General tab and scroll down to the “Media Manager/Media Grid Enhancements” section, then uncheck the first two boxes to prevent MLA from downloading any JavaScript to your browser. Scroll to the bottom and click “Save Changes” to save your work. If that works, let me know.
Of course, you can continue to use the earlier MLA version. You should know, however, that older MLA versions do not work well with the new WP 4.0 Media Grid view in the Media/Library submenu. That’s why I really want to get the latest version working for everyone.
If you can give me a list of the other plugins you have installed and activated I can try to reproduce the problem. Thank you again for your patience and your help.