One of the root certificate of COMODO has expired at 30 May.
That’s why we get error from your nuget server.
Here you can see the expired certificate.
Can you replace the expired certificate with new one ?
Sorry for this, was confused by this article – https://thesslonline.com/blog/sectigo-addtrust-external-ca-root-expiring-may-30-2020 – that no action is required, and SSL monitoring tool told me all is ok yesterday. I’ve just reissued the cert and updated it.
Problem has solved thank you for your support.