I tried to reproduce your issue on my machine by modifying the Fiddler's script on an old version of Fiddler to add the custom button on booting. Then I executed an update to the latest version of Fiddler. The custom button in the toolbar showed up again.
Could you, please, elaborate on how exactly you updated Fiddler to the latest version and from which version you did the update.
Please, let me know if the changes in the script which you had made by following the instructions in the post are still there or if the script was reset to the default.
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