Generally, Fiddler doesn't automatically attempt to reestablish itself as the Proxy as this could lead to an infinite loop. Something like this might be caused by VPN Software, Firewall Clients or even Group Policy/Login scripts. In order to troubleshoot this I'll need more information. Can you provide the following information?
1. Screenshot of the Proxy Settings when Fiddler is running?
2. Screenshot of the Proxy Settings when Fiddler is not running?
3. Screenshot of the Fiddler Connections Tab in settings? To find this go to Tools -> Options -> Connections
4. Output of the Fiddler Log Tab? To get this, select all items in the Log Tab and right-click -> Save to file.
Once I have this information I will be able to provide a more thorough response. Thank you and I look forward to your reply.
Eric R | Technical Support Engineer
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