Due to a bug in the auto-updater, some clients might experience a crash when attempting to update from an older version to version 1.0.1.
To resolve the issue when prompted for an update, choose NO and wait for approximately a minute (depending on your connection speed) before relaunching the Fiddler Everywhere client. After the delayed restart, the newest version will be installed.
FYI: Updating from version 1.0.1 (or newer) to the latest version now works as expected. To update from version 1.0.0 to latest please use the solution mentioned above.