Parser Error: Unknown server tag asp:ScriptManager

Problem

You see this message when attempting to run your application:

Server Error in ‘XXX’ Application.

Parser Error

Description: An error occurred during the parsing of a resource required to service this request. Please review the following specific parse error details and modify your source file appropriately.

Parser Error Message: Unknown server tag ‘asp:ScriptManager’.

This error occurs when you overwrite your application’s Web.config file with an older version that does not contain the appropriate information to enable Ajax functionality in your application.

Solution

Step 1: Make a copy of your application’s Web.config file.

Step 2: Create a new application using Iron Speed Designer V4.2 or later. This will create a good Web.config file.

Step 3: Copy the Web.config file from the new application folder into your application folder, overwriting the older or mis-configured Web.config file.

Step 4: Apply any necessary changes to the new Web.config file that may be present in your older Web.config file (from the backup copy).