Assembly "AjaxControlToolkit, Version=3.5.40412.0, Culture=neutral, PublicKeyToken=28f01b0e84b6d53e" does not contain a script with hash code "de1feab2"

Hi All.

I've upgraged my website from .net framework 3.5 to 4.0 about one year ago but I kept the AjaxControlToolkit 3.5
Now from last few days I was getting the errors on our site and they are logged by Exception layer
Error is:-
Assembly "AjaxControlToolkit, Version=3.5.40412.0, Culture=neutral, PublicKeyToken=28f01b0e84b6d53e" does not contain a script with hash code "de1feab2"

To take care of this I've updated the AjaxControlToolkit from 3.5 to 4.0
Now I am getting the following error:-
Could not load file or assembly 'AjaxControlToolkit, Version=3.5.40412.0, Culture=neutral, PublicKeyToken=28f01b0e84b6d53e' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)

These two errors have one thing common. We are getting the following QueryString and this was the only reason for the crashes:-
_TSM_HiddenField_=ctl00_sm1_HiddenField&_TSM_CombinedScripts_=%3b%3bAjaxControlToolkit%2c+Version%3d3.5.40412.0%2c+Culture%3dneutral%2c+PublicKeyToken%3d28f01b0e84b6d53e%3aen-US%3a065e08c0-e2d1-42ff-9483-e5c14441b311%3ade1feab2%3af9cec9bc%3a8ad18101