I saw that many people have been having some issues with jQuery.BlockUI versions after upgrading to WordPress 3.6. So did I. Having looked into it, this is what I discovered (and fixed) in my setup.
1. WordPress 3.6 uses the newer (latest) version of jQuery, which is 1.10.2.
2. Lots of plugins use a function call jQuery.BlockUI which requires jQuery 1.2.3 or above.
3. Now, 1.10 IS higher than 1.2, but older versions of the jQuery.BlockUI code used by plugins have a little 'buglet' - the code tests for versions BUT drops the trailing '0', so instead of asking is 1.10 greater than 1.2, it tests if 1.1 is greater than 1.2, and so it fails.
4. This is NOT a WordPress problem - WordPress 3.6 merely uses the latest version of jQuery. Previous versions up to 1.9 would obviously pass the buggy test in jQuery.BlockUI code.
5. The answer is NOT to downgrade jQuery in WordPress. Well - that might work, but don't do it!
6. The answer is to get your PLUGIN provider to update their plugin.
7. My plugin used a file called wf-an-jqery-plugins.js. Inside that file was a chunk of jQery.BlockUI code that said it was v2.39.
8. I went to the following link for the latest jQuery.BlockUI code (v2.64) and then just replaced the v2.39 code in that plugin file with the code from this link. Hey presto, it all works again: http://malsup.github.io/jquery.blockUI.js
9. So, folks, find out which plugins you have that are using OLDER jQuery.Block UI code and have your plugin provider update it - and in the meantime if you understood what I wrote above, perhaps you can fix your plugin(s) yourself.
I hope this helps.