What worked for me is to reset the fiddler https certificate and recreate it.
Fiddler version V4.6XXX
Fiddler menu-> Tools-> Telerick Fiddler Options...
Second tab- HTTPS-> Action -> Reset All Certificate
To solve this, I had to delete manually the folder ../Documents/Fiddler2 which for whatever reason was not getting removed even after uninstalling and then reinstalling Fiddler. I'm on Fiddler 5.0, build from 03 November 2020.