I just resolved my issues when i updated to 2.8.
First issue being something with my taxonomy object, giving an error in /wp-admin/menu.php (line 38), adding (array) casting in the foreach there solved the problem for now.
foreach ( (array)$wp_taxonomies as $tax ) {
Second issue was indeed the drag n drop widget interface. I wasn’t able to drag or drop, just the cursor toggled to dragmode.
I traced this issue back to my own ecommerce plugin, which makes use of the jQuery UI library.
Commenting out my own wp_enqueue_script
lines for these library files, and making my own script dependable on the jquery-ui-core
file fixed everything.
I recommend everybody to check the plugins they use, to make sure none of them actually include their own version of the standard WP Script files (jQuery, jQuery UI, Scriptaculous, etc).
And as a personal note: I agree with azaozz when he claims the issue can often be traced back to the plugin developers.
As an active webdeveloper, I often make use of open-source CMS software, and one of the most important things to check when creating modules (Drupal :’)) or plugins (WordPress), is to make sure you use to apropriate (standard) methods and libraries of that system, instead of just adding your own versions to make it work.
Hope to have helped someone!