There isn't a solution can solve all the various cases, but if you get this errors:
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from one day to another
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without any changes in the code
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you get both error messages: 'WebForm_PostBackOptions' is undefined and 'sys' is undefined
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in your dev environment everything works perfectly
well, maybe you have installed the Microsoft ASP.NET 2.0 AJAX Extensions and then you have modified the date of your server, setting it to a date before the installation day.
if it doesn't solve your problem try to look at this page:
http://forums.asp.net/t/1040236.aspx?PageIndex=8