There has been a unpublished local feature of the mailman lists service running on https://lists.inf.ed.ac.uk which has changed. The short version is that if you used to deliberately use the URL https://lists.inf.ed.ac.uk/mailman/… to log you into list, you should now use https://auth.lists.inf.ed.ac.uk/mailman/…
Now a bit more detail and history. The original web interface to the list management pages were only available on the HTTP website http://lists.inf.ed.ac.uk/, and any authentication used mailman’s built in mechanisms. Later, as a convenience to local Informatics users, the HTTPS version of the site, https://lists.inf.ed.ac.uk, used weblogin.inf.ed.ac.uk and Cosign to authenticate you, and if you were on (or managed) the list using your UUN@inf.ed.ac.uk email address, then you’d get the appropriate access*. This saved you having to remember your mailman password, and used the more secure HTTPS protocol.
This feature was not widely announced, as its usefulness is limited. It didn’t work for non-Informatics members of lists, and also doesn’t work if you are Informatics, but are on the list with an address other than your UUN@inf.ed.ac.uk style address*.
Nowadays modern web browsers will prefer to use the HTTPS versions of websites, rather than the HTTP version. This has been leading to more problems for users of lists.inf.ed.ac.uk, as non-Informatics people were unable to authenticate and hence use the https://lists.inf.ed.ac.uk URL. If they forcibly used the HTTP version, then they would be OK (apart from HTTP being a poor choice for things asking for passwords!).
So we have now removed the Cosign authentication from the https://lists.inf.ed.ac.uk/ version of the URL. It now behaves the same way as the HTTP version of the site, but over the more secure HTTPS connection.
For those that did use the Cosign feature of the previous version of the HTTPS site, this has now moved to the new URL https://auth.lists.inf.ed.ac.uk/… however, as before, this feature is only really of use for Informatics users who manage (or are on) lists with their UUN@inf.ed.ac.uk email address*.
[*] This is not strictly true, your @ed.ac.uk email address may also work, depending on what is registered as your primary contact email address.