One of the most common reasons for "Error opening file for writing" errors is the lack of administrative rights and limited file/folder access. Ensure that you are using the official download link to obtain the latest version of Fiddler Everywhere and then run the application installation as an administrator.
No joy. Second time that this has happened at it always starts with one of your updates corrupting the install and prevents a reinstall.
The application has admin permissions to install.
Very frustrating, i need to be able to rely upon this tool and instead you uninstall it without my permission!
Update: The issue was apparently related to an antivirus program (Avast) that was force-killing Fiddler Everywhere on startup.
Downloaded, again, ran install. Same error as last time
Error opening file for writing
Clearly i need to turn off your automatic updates some how.
I cannot find fiddler listed in windows add remove apps