In order to help you, I'll need to gather a bit more information. Can you tell me which Certificate Maker are you currently using? You can check that from "Tools > Options > HTTPS > Certificates are generated by ....". Are you using the default CertEnroll engine or are you using something else? Alternatively, there might be a preference setting, which is altering the certificates validity period. Can you type "about:config" in QuickExec and take a scееnshot of your preferences and attach it here?Alternative solution to download and install the Bouncy Castle Cert Maker plugin from here: https://www.telerik.com/fiddler/add-ons. It's under the name "CertMaker for iOS and Android". Once you install, the issue should be resolved.
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Tools -> Options -> HTTPS -> Actions -> Reset All certificates
After a series of pop-ups, a new root cert is generated that has a future expiry date.