Description:  If you have an update panel with a validator of any type associated with it, it will cause a postback on the first attempt of the button click, but on subsequent clicks of the button, it will fail to postback to the server.

Reason: This is caused by the javascript not allowing the postback to be passed to the server because it cannot recognize the controls in the update panel on the second attempt. 

Fix: To get around the javascript, you can force a postback to always occur and bypass the javascript on the client's machine.  The AjaxToolkit is eventually going to be updated to fix this problem, but for now, you can add the following attribute to your Validator and it should work:

EnableClientScript="false"