copying from https://groups.google.com/forum/#!topic/httpfiddler/nmgr5kZ6RkU
You don't need to set registrationhostname when using a remote device (and you should undo that using about:config or Fiddler will break the next time your PC gets a new IP).
You have correctly set "Allow Remote Connections" and your firewall is properly configured; we know this because you're able to see traffic from your laptop.
Would I be correct in guessing that you're a Microsoft employee who is connected to CorpNet?
The problem you're having is almost certainly caused by use of IPsec, which prevents TCP/IP connections from non-domain devices to domain-joined PCs. You need to go to ITWeb and search for "IPsec boundary exemption" which will lead to a form which you can use to set your desktop PC as exempt from IPsec rules. Or you can use one of the various "Fiddler as a service" servers that some teams around the company have configured (which are already boundary-exempt).
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