nice!! Thanks very much!! I think that explained, as my access on 30 May was still OK, until today, 31st May the issue comes.
Also, "A more modern browser would have the USERTrust root already installed and trust it without needing to rely on the older AddTrust root." --> I think that is why accessing the website via Chrome gives no warning. I notice that the Certificate chain display from the browser is showing
COMODO RSA Certification Authority
However, I'm still not sure how to clear the warning message though