I'm having the exact same problem.
I have tried to reset IIS and all that but it doesn't help me.

This is how my problem appears:

My calendar works until I do a postback. then it doesn't works any more and I get the following javascript error: Date.parseLocale is not a function

If I clear the textbox the extender is attached to at the postback
tbxDate.Text = string.Empty

then it works! but this isn't how it should work.
 

someone with any more suggestion?