This only occurs when double-clicking a SAZ file from Explorer when Fiddler isn't open.

---------------------------
Script Failure
---------------------------
Failed to load script.
 Microsoft.JScript.JScriptException: Type mismatch

   at Microsoft.JScript.Convert.CoerceT(Object value, Type t, Boolean explicitOK)

   at Fiddler.ScriptNamespace._Startup.Startup()

   at Microsoft.JScript.Vsa.BaseVsaEngine.Run()
Microsoft.JScript.JScriptException: Type mismatch

   at Microsoft.JScript.Convert.CoerceT(Object value, Type t, Boolean explicitOK)

   at Fiddler.ScriptNamespace._Startup.Startup()

   at Microsoft.JScript.Vsa.BaseVsaEngine.Run()   at Microsoft.JScript.Vsa.BaseVsaEngine.Run()

   at Fiddler.FiddlerScript._LoadScript(String sScriptFilename, Boolean bPlaySounds) in C:\JenkinsHome\jobs\FiddlerReleaseBuild\workspace\Fiddler2\Common\Application\Scripting\Scripting.cs:line 315

   at Fiddler.ScriptBase.LoadRulesScript(Boolean bPlaySounds) in C:\JenkinsHome\jobs\FiddlerReleaseBuild\workspace\Fiddler2\Common\Application\Scripting\ScriptBase.cs:line 198
---------------------------
OK   
---------------------------

This reproduces even when the FiddlerScript is basically empty, like so:

import Fiddler;

class Handlers
{

}

Sadly, because Fiddler's code has been obfuscated, it's difficult to see what caused this regression.