Reattach Fiddler as system proxy if unexpectedly detached; see https://feedback.telerik.com/fiddler/1410460-the-system-proxy-was-changed-click-to-reenable-capturing for discussion.

reattachFiddlerscript.js

// Click Rules > Customize Rules. Scroll to OnBoot() and inside the function add:

  static function OnBoot() {
     FiddlerApplication.oProxy.add_DetachedUnexpectedly(DoReattach);
  //...
 

// Just before the OnBoot function, add the following new functions:
static function DoReattach(o: Object, ea: EventArgs) {
  FiddlerObject.UI.sbpInfo.Text = "Scheduling automatic reattach at " + new Date();
  // Reattach after 1 second; waiting 1 second increases the odds that whatever
  // process was messing with the system settings is done doing that.
  ScheduledTasks.ScheduleWork("reattach", 1000, innerReattach);
}

static function innerReattach()
{
   FiddlerApplication.UI.actAttachProxy();
}

static function OnRetire()
{  // Detach this event handler to avoid memory leaks and duplicate processing.
   FiddlerApplication.oProxy.remove_DetachedUnexpectedly(DoReattach);
}