Could not load file or assembly 'System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependenc

This error ocuurs in server if you do not run the .msi installer on the server.

If you didn't, you need to copy the System.web.extension dll.

Solution to that extension conflict for system.Web.Extension

If in web.congig that extension conflicts ocuurs with dll of system.web.extension you can resolve it this stuff after system.web section completion.
<runtime>

<assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">

<dependentAssembly>

<assemblyIdentity name="System.Web.Extensions" publicKeyToken="31bf3856ad364e35"/>

<bindingRedirect oldVersion="1.0.0.0-1.1.0.0" newVersion="3.5.0.0"/>

</dependentAssembly>

<dependentAssembly>

<assemblyIdentity name="System.Web.Extensions.Design" publicKeyToken="31bf3856ad364e35"/>

<bindingRedirect oldVersion="1.0.0.0-1.1.0.0" newVersion="3.5.0.0"/>

</dependentAssembly>

</assemblyBinding>

</runtime>