While the error is happening during activation of SmarterMail, the process that is failing looks to be a utility in Plesk that they use to make configuration changes. I think your best bet might be starting with Plesk to see if they can point to more detailed log locations which may provide hints as to why flipping that switch is failing. The other thing that might help is attempting to make that switch, then checking the Event Viewer in Windows for any Errors/Warnings pertaining to that exe. I hope that helps!
Kyle Kerst
Acting IT Manager
SmarterTools Inc.
www.smartertools.com