shell_exec() fails to execute on IIS 10 server

Created by: pedwik

I get this error when trying to import a repository: APPLICATION ERROR #plugin_SourceViewVC_run_svn shell_exec() failed to execute Subversion. Please use the "Back" button in your web browser to return to the previous page....

Setup: OS: Window Server 2016 (VPS) MantisBT version: 2.6 Database: Sql Server 2014 Express PHP: 7.1.9

Settings made in Source plugin configuration UI: SVN: Path to binary: C:\csvn\bin SVN: Command Arguments: Empty SVN: Trust All SSL certifcates: True SVN: Use Windows Start : True (tried both)