System.Web.Extensions version error

Problem:

when trying to access a SharePoint page (can repro with plain ASP.NET too), this exception got thrown:

An error occurred during the processing of . (0): error CS1705: Assembly 'Microsoft.Whatever' uses 'System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' which has a higher version than referenced assembly 'System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'

Solution:

in affected web.config, replace

"System.Web.Extensions, Version=1.0.61025.0"

with

"System.Web.Extensions, Version=3.5.0.0"

and iisreset.