Created Unassigned: How to solve this error: ASP.NET Ajax client-side framework failed to load. [27677]

I have been struggling from the last 2 days, i have been trying the solutions provided all around the Google but have tough time in fixing the issue.

I am using ToolkitScriptManager and ModalPopupExtender in my Login Page but when the Login Page is loaded i get 3 Javascript errors and it makes ModalPopupExtender useless.

The errors are:

ASP.NET Ajax client-side framework failed to load.

'Sys' is undefined

'Sys' is undefined

This works fine in my local machine and Production but in Development Server, I have this problem. Please help.