Recently one of our customers had an issue when migrating from ASP.NET 1.1 to ASP.NET 2.0. The issue came up particularly in the deployment server where any page that had validations raised the error ‘Webform_PostBackOptions’ is undefined.
If you had worked with ASP.NET 1.1 you would recollect there was an aspnet_client folder that contains all the scripts that help in client side validation as well as raising the postback events.
In ASP.NET 2.0, the “WebResource.axd” file is the handler that generates all the client side scripts which were earlier processed by the aspnet_client folder.
Coming back to this particular customer scenario, they were running URL Scan utility on their server http://learn.iis.net/page.aspx/473/using-urlscan
URL Scan is an useful free utility that can be installed on servers that allows you to restrict the file extensions served by the IIS. This way, potentially harmful requests can be denied and thereby injection attacks avoided.
However, the WebResource.axd handler had to be manually added as an extension to be allowed and thereafter this error went off.
There are many more causes of this particular error and I found that it has been discussed enough in forums. Here below is one of them for your reference