Powershell Add-Type not working

I have installed WinSCP v.5.7.7. in C:\Program Files (x86)\WinSCP\ and unlocked the WinSCPNet.dll as documented, but when I enter Add-Type -Path "WinSCPnet.dll" in Powershell it still fails:

Add-Type : Could not load file or assembly 'file:///C:\Windows\system32\WinSCPnet.dll' or one of its
dependencies. Operation is not supported. (Exception from HRESULT: 0x80131515)
At line:1 char:1
+ Add-Type -Path "WinSCPnet.dll"
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : NotSpecified: (:) [Add-Type], FileLoadException
+ FullyQualifiedErrorId : System.IO.FileLoadException,Microsoft.PowerShell.Commands.AddTypeComma
nd

This is a new Windows 2012R2 server. I need this to work as I have to convert all of my VBS scripts that call WinSCP to Powershell scripts ASAP.

Duplicate post:
https://winscp.net/forum/viewtopic.php?t=23808