Could not load file or assembly 'System.Net.Http

Thanks to @Jehof:

Reason

Web.config had redirect of System.Net.Http from 4.0.0 to 4.1.1, and GAC had only assembly version 4.0.0.

<?xml version="1.0" encoding="utf-8"?>
<configuration>

  <runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
...
      <dependentAssembly>
        <assemblyIdentity name="System.Net.Http" publicKeyToken="b03f5f7f11d50a3a" culture="neutral" />
        <bindingRedirect oldVersion="0.0.0.0-4.1.1.1" newVersion="4.1.1.1" />
      </dependentAssembly>
...
    </assemblyBinding>
  </runtime>


</configuration>

Solution

Removing this redirect solved the problem.

Root cause

I think, I had this assembly redirect in Web.config, because I installed System.Net.Http via nuget at some point, and then uninstalled it. However assembly redirect was not removed automatically.