Javascript – Invalid postback or callback argument error

It's a known problem with the AJAX CascadingDropDown extender.

In order for it to work properly, you need to disable event validation.

Here's a thread discussing the issue: http://forums.asp.net/t/1032053.aspx

I had the same issue, that's why i ditched using the CascadingDropDown extender and just used regular client-side dropdownlist's and some jQuery.

You've got two options:

1. Ditch the AJAX CascadingDropDown, replace with regular dropdowns, calling web service on client click with jQuery/javascript.
2. Disable event validation on the page. (not recommended).

Event validation prevents the state of the page being tampered with between requests. Unfortunately, the AJAX CDDL does exactly this, for whatever reason.

It's not something you should disable just to get the CDDL working, as it will affect the entire page and can cause security issues.

My advice, bite the bullet - ditch the CDDL and replace with jQuery.

This has to be one of the most frustrating error messages in .NET, but once you get a feel for what's going on, it makes sense. .NET likes to know EVERYTHING that's going on. It keeps track of all the elements that it has placed on the page. Along those same lines, .NET gets offended when it receives input from something it didn't know about. In your case, it sounds like, at the time you click on the LinkButton, .NET doesn't think it should be there. In my experience, there are two likely reasons for this:

1. You're doing to client-side wizardry that is creating new inputs or cloning existing inputs.

2. While the form submission is being processed, .NET does something to the LinkButton which causes it to be no longer available. Some examples of this I've run into are when your LinkButton is dynamically created in the backend or you're using UpdatePanels and their content get changed during the form's submission.

Basically, I believe if you step through the form submission code and watch that LinkButton, you'll see .NET forget about it, which understandably triggers this "Security Exception" when the LinkButton is clicked.