Could not load file or assembly Operation is not supported. (Exception from HRESULT: 0x80131515)

Right click on the Dll and unblock it.

During my research I found that this may happen when you downloaded a test in a ZIP from the internet and the advice was to unblock the ZIP. But I copied the files over the network from my developer system to the test PCs and the files were not blocked.

Searching more for the error code 0x80131515 I found this answer in the MSDN forum.

So I edited the file

C:\Program Files (x86)\Microsoft Visual Studio 11.0\Common7\IDE\CommonExtensions\Microsoft\TestWindow\vstest.console.exe.config

and added the entry <loadFromRemoteSources enabled="true" /> under the runtime tag and voilá, the tests run fine without any issues from a scheduled task.

Maybe this helps other users who also run into this strange issue.

As previously stated, a Zip file is blocked when downloaded from an untrusted site. If you are confident the site is reliable you can prevent this behavior by adding it to your trusted sites list in Internet Explorer -> Tools Menu -> Internet Options -> Security Tab -> Trusted Sites. Future downloads will not be marked as blocked.

For blocked files that have already been extracted, run this at a Powershell prompt to unblock all the files in the current folder and subfolders:

Get-ChildItem *.* -Recurse | Unblock-File