A few websites I manage are running then.ly’s All-in-One Event Calendar plugin. Ever since updating those sites to WordPress 3.6, a “BlockUI” error message kept popping up in admin on virtually any admin page, which was rather annoying.
Their help site doesn’t really offer much in the way of help on the subject, but after a lot of trial and error (and online research) I have discovered that you need to upgrade the plugin to the latest version, which is 1.10-standard at the time of writing.
Ordinarily, you would just update the plugin in WordPress but the plugin’s authors seem to have had something of a rebrand (changing from “then.ly” to “time.ly”), and as far as I can tell they just created a brand new plugin based on the old one, rather than updating the existing plugin in the WordPress plugin directory with the new version.
In order to fix the BlockUI error message and upgrade to the latest “time.ly” version, you will need to download the Timely Updater plugin (also available in the WordPress plugin directory), then once it is installed and activated you need to go to Events > Timely Updater from the sidebar in admin.
I would advise making a backup of your WordPress database beforehand, because if the update screws up the plugin for whatever reason, it will kill the plugin and potentially take any posts linked to the events with them! I recommend doing a backup in phpMyAdmin.
Once the plugin has been successfully updated, it should look like this in your plugins list: