Everyone thanks so much for your reply!

It is clear to me now that the SSL certificate sent by Bitpay is not accepted by Electrum and it's not possible to manually edit this. It is something Bitpay has to fix.

In the mean time one can use a decoder (like https://alexk111.github.io/DeBitpay/ ) to extract the payment details. Alternatively you can tell Bitpay you're using Samourai Wallet (which does not support payment protocol), Bitpay should then show the details to manually make the payment.