The ModalPopup cannot be used together with an UpdatePanel and one textbox. This is because during the postback, the value of the textbox will be changed and added a comma.
That is, if we use a ModalPopup to have a form and then we add a database, the value "Antonio" in the text box will be entered as "Antonio."

I've been researching a bit and this bug is even launching this version. To counter this I'm using the stable 30 512 May 2009 - http://ajaxcontroltoolkit.codeplex.com/releases/view/27326

One detail I noticed was that the ModalPopup OkControlID on the property. If we define the event does not occur (example: If this button is add the data to the database, the panel controlled by ModalPopup disappears but not to add the event occurs).

Therefore it is best not declare anything on this property, it works without problems the same.