官方版
Fiddler Crashes on Startup with an unhelpful message box
If you see this message box when starting Fiddler:
…it generally means that your .NET Framework installation is corrupt. If you uninstall and reinstall the .NET 2.0 Framework, the problem is usually resolved.
谷歌翻译版
Fiddler在启动时崩溃,并显示无用的消息框
如果在启动Fiddler时看到此消息框:
…这通常意味着您的.NET Framework安装已损坏。 如果您卸载并重新安装.NET 2.0 Framework,则通常可以解决该问题。