Missing Microsoft.Build.Tasks.v4.0.dll on visual studio online tfs build - tfs

Question

I am trying to build my project with visual studio online.
I am getting the following error.
C:\a\src\.nuget\nuget.targets (71): The task factory "CodeTaskFactory" could not be loaded from the assembly "C:\Program Files (x86)\MSBuild\12.0\bin\amd64\Microsoft.Build.Tasks.v4.0.dll". Could not load file or assembly 'file:///C:\Program Files (x86)\MSBuild\12.0\bin\amd64\Microsoft.Build.Tasks.v4.0.dll' or one of its dependencies. The system cannot find the file specified.
This is coming from the nuget.targets file in my solution.
<UsingTask TaskName="SetEnvironmentVariable" TaskFactory="CodeTaskFactory" AssemblyFile="$(MSBuildToolsPath)\Microsoft.Build.Tasks.v4.0.dll">
<ParameterGroup>
<EnvKey ParameterType="System.String" Required="true" />
<EnvValue ParameterType="System.String" Required="true" />
</ParameterGroup>
<Task>
<Using Namespace="System" />
<Code Type="Fragment" Language="cs">
<![CDATA[
try {
Environment.SetEnvironmentVariable(EnvKey, EnvValue, System.EnvironmentVariableTarget.Process);
}
catch {
}
]]>
</Code>
</Task>
</UsingTask>
I have updated nuget.exe to latest and have not changed anything in nuget.targets.

Answer 1

The problem was that some of the class libraries in the solution had the default value :
ToolsVersion="12.0"
changing that to
ToolsVersion="4.0"
made it work on TFS online

Answer 2

I ran into this after upgrading a project to .NET 4.5.2. Seems to be a conflict between the .NET 4.5 point releases and the old way to do NuGet package restore (MSBuild-Integrated package restore vs Automatic Package Restore).
I was able to solve the issue by migrating NuGet to the new way of doing package restore: http://docs.nuget.org/consume/package-restore/migrating-to-automatic-package-restore
More info: http://blog.davidebbo.com/2014/01/the-right-way-to-restore-nuget-packages.html