Suddenly emails are no longer downloaded from Plusnet to Thunderbird client

[#1]

For years I have been download/deleting emails from my Plusnet email address.
This stopped working a few days ago.
The attached images are
(1) An AddException error message from Thunderbird
(2) A certificate from ViewCertificate

I am confused as to know what to do?
The certificate has a validity entry that implies it should have stopped working August 19th ? but it worked until recently.

I have logged another question on the Mozilla Thunderbird site, in case they could help, but anyone have any pointers?

Does the certificate look valid or problematic?   How do I obtain a valid one?

Thanks & regards
Barry

PS
Probably not relevant to the above, just FYI, but all my various email/gmail addresses are auto forwarded to plusnet.
I then use Thunderbird to download and delete them, as they are filtered into a few dozen different subject folders.

[#2]

@bathsheba The certificate has a validity entry that implies it should have stopped working August 19th ? but it worked until recently.

The certificate seems to be valid FROM 19th Aug 2024 TO 18th Aug 2025 so is fine.

What's the error from Tbird ?

[#3]

Thanks.

Silly of me to miss that.

This support page stopped me uploading the error message - each upload seemed to overwrite the previous one.

I added the message below.

I have not pressed the 'confirm exception' button yet, lacking knowledge and confidence to do so.

[#4]

Looks like there may be a problem in the certificate chain. I'm afraid that's not my area of expertise.im not at home at the moment but I'll try my tbirdvconfig tomorrow and see if it reports a similar issue

[#5]

What version of Thunderbird are you using? Outdated root certificates can see these sorts of errors with "unknown issuer" or trust problems.

It should be relatively safe to allow the exception if you really need to get to your emails urgently. (it is a reversible change)

[#6]

@bathsheba just tested on my Windows 11 system using Thunderbird Vn 128.2.1esr and no problems showing.

As @RPMozley says , it may well be an intermediate/root certificate issue and there should be no problems in adding the Exception

[#7]

I just ran a check over on SSL shopper's test tool (https://www.sslshopper.com/ssl-checker.html#hostname=mail.plus.net:995), and all looks fine