BT OpenZone Certificate Expired?

[stockhausen]

I seem to have a problem with either Firefox or BTopenzone.
When I am working away from home, I sometimes connect to BT Fon.
Yesterday I got the following message:

Your connection is not secure

The Owner of www.btopenzone.com has configured their website improperly. To protect your information from being stolen, Firefox has not connected to this website.​

I am then offered two options, "Learn more..." or "Advanced".

"Learn more" adds:

support.mozilla.org uses an invalid security certificate.

The certificate is only valid for the following names:
btopenzone.com, www.btwifi.com, reg.btwifi.com, info.btopenzone.com, cdn.btwifi.com, btwifi.com, btwifi.co.uk, www.btopenzone.com, my.btwifi.com, my.btopenzone.com, info.btwifi.com, www.btwifi.com

Error code: SSL_ERROR_BAD_CERT_DOMAIN​

"Advanced" adds:

www.btopenzone.com uses an invalid security certificate.

The certificate expired on 05 September 2016 00:59. The current time is 07 September 19:22.

Error code: SSL_ERROR_EXPIRED_CERTIFICATE​

Any suggestions as to what might be causing this, Firefox or BT OpenZone?

 

[stockhausen]

Did you get the error as soon as you connected or after you had been connected for a while?

As soon as I entered the URL and hit [ENTER], before entering the login details.

 

[stockhausen]

Incidentally, this happens with both MSIE 11 & Firefox 48.0.2.

 

[stockhausen]

{SNIP}
For some reason, the browser isn't always redirected to the correct landing page. Bookmark https://www.btopenzone.com:8443

Many thanks, that seems to work.

Why is that the case and why did it change recently? Is BT or Mozilla to blame?