With regard to Symantec, you'll have to talk to them about why their software is so false-positive prone. The AV market as a whole is biased toward false-positives for competitive reasons.
With regard to your issue, it sounds like you may be having issues with the generated certificates. You might try temporarily disabling HTTPS decryption, choosing "Remove Interception certificates" and then re-enabling HTTPS decryption.
After this, you should watch for any errors on Fiddler's LOG tab.
You may also find Fiddler's Help > Troubleshoot Filters feature useful.
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