I have an ASP.Net 4.0 application in which contains a web form containing a ScriptManager, UpdatePanel and some controls, including  a WebDropDown. This form has been in production for some time now with no issues. The app is using 2010.3

I recently tried to add another field to the form (textbox), and after compiling and executing the code on my VS2010 PC when I attempt to post the page back I started getting this error:

"Microsoft.JScript" runtime error: Sys.WebForms.PageRequestManagerParserErrorException: The message received from the server could not be parsed."

I have tried removing my changes to the page, but the error still comes up. I even pulled from the production server the same web form in production (aspx, cs and designer files) and replaced the existing files of my project, but I still get the same error.

Testing other web forms in my application using AJAX, they work fine, so I assumed it must be a problem with the specific form.

So I stripped down my page and left only the WebDropDown and the button causing the postback and tested. The problem occurs with the WebDropDown and goes away without it. I went one step further and left the WebDropDown with no data, and the problem did not occur. Loading some dummy data manually in the code behind (as opposed to data from the db) also results in the problem not happening. So my next assumption was that this is a data related issue (hence why the problem did not happen in production).

To test my theory I downloaded the prod database and used it on my machine; the problem occured, so my theory is wrong.

I am running out of ideas, so please if you have ever encountered such an issue I would appreciate some help.