After downloading LEAN and opening it in Visual Studio (I'm still on VS 2017), when you attempt to build it for the first time, you may encounter this error:

error MSB3821: Couldn't process file WinForms\LeanWinForm.resx due to its being in the Internet or Restricted zone or having the mark of the web on the file

The fix I found for this is to locate the file in the code base (...\Lean-master\UserInterface\WinForms\), right click on the file name (LeanWinForm.resx), and select properties.  At the lower right hand corner of the properties panel, you will see a checkbox for the option 'Unblock'.  Click on the box to check it, save the properties, and then rebuild LEAN.  This should allow you to build without the error.

Remark: Once you have checked this option, if you open the properties again, the option box will not be present.  

Reference:  Link below on StackOverflow, 2nd answer.