Change 26814 seems to have introduced a regression that can be seen by opening the /ToolkitTests/Manual/Validation.aspx page. Previously (and as intended), loading the page and clicking the Postback button would display the validation failures. In the 10920 release of the Toolkit, it displays the ConfirmButton prompt and then performs a postback if OK is clicked. This appears to be related to the introduction of the this._oldScript variable in change 26814 and specifically the way it's not used by the ConfirmOnFormSubmit scenario in _onClick. A very brief look suggests that moving the execution of that code out a level so that it applies to both cases results in correct behavior, but I haven't taken the time to completely understand this change, so that may not be the correct fix.
Comments: This ticket was filed for a pre-15.1 version of AJAX Control Toolkit. If this is still an issue in v15.1 or later, please create a new inquiry.
Comments: This ticket was filed for a pre-15.1 version of AJAX Control Toolkit. If this is still an issue in v15.1 or later, please create a new inquiry.