asp.net: Invalid postback or callback argument

in you aspx file you should put the first line as this :

<%@ Page EnableEventValidation="false" %>

if you already have something like <%@ Page so just add the rest => EnableEventValidation="false" %>

I recommend not to do it.

If you look at the first lines of text you can glean what your error is.

this feature verifies that arguments to postback or callback events originate from the server control that originally rendered them

You're dynamically editing the lstProblems dropdown, so when you post back ASP.NET says "Warning! Invalid entries in the dropdown!" and freaks out throwing that error. You have to determine if turning off event validation is an OK solution, but I would research it before doing it, since the idea behind it is to make your site more secure for free.

Here's another stackoverflow answer that does a much better job explaining what to do than me: Invalid postback or callback argument. Event validation is enabled using '<pages enableEventValidation="true"/>'

If you have code in your Page_Load() event. Try adding this:

if (!Page.IsPostBack)
{ 
//your code here 
}