This may be completely unrelated, but...
A lot of sites use LetsEncrypt to get free SSL certificates (which you need to make a website secure i.e. HTTPS) which includes ukcaving (assuming I am reading the certificate stuff correctly).
Your computer or (in the case of Firefox only) your browser has a list of 'root' certificates which they implicitly trust; these have been produced by various companies that sell or otherwise are trusted to produce SSL certificates for websites. Modern operating systems will update this list, but older ones do not.
Let's Encrypt are unusual because they will give you certificates for free. However, they are still trusted because they still use a set of tests to ensure that you really do control the domain (e.g. ukcaving.com) that you want to identify yourself as. They have a 'root' certificate that they can use to sign your certificate for (for example) ukcaving.com. Then, when a visitor comes onto your website, they get your certificate saying 'this is ukcaving.com'. But your browser doesn't implicitly trust this certificate; instead it looks to see who has signed it using their certificate and (in the case of most modern operating systems) they see 'ah, Let'sEncrypt have used their root certificate, which we hold a copy of the public part of in our root certificate store, to sign that certificate to indicate that this person really does control ukcaving.com' and so your browser indicates 'this really is ukcaving.com according to someone we trust'. This is an oversimplification, and there are actually more layers of certificates, but that doesn't really matter.
This is all to stop someone redirecting your internet traffic and then pretending to be ukcaving, sucking up your login details and posting inflammatory comments about access or something.
The issue is that when LetsEncrypt started, not that many years ago, nobody trusted their 'root' certificate - which is to say that the public part of it was not included in root certificate stores in your operating system (because it was new). Nowadays it is included (in things since Android 7ish, Mac OS 10.2ish, Windows Vista onwards etc) so computers trust it implicitly, but in order for it to be accepted back then it was signed by another companies root certificate which was widely trusted, and so the Let's Encrypt certificate became trusted in turn.
The problem is that the other company's root certificate expired on the 30th September 2021, which means any older device (not running Firefox) which doesn't have Let's Encrypt's root certificate in its root certificate store no longer trusts a lot of websites. This includes Mac OS <10.2ish, Windows XP SP3 or before (without root certificate updates) and things like smart TVs and Playstations. Android has a weirdness where it trusts the root certificate even though it has expired, so shouldn't have a problem.
Basically, if you are on an old, old operating system and can't access UKCaving without dodging various security warnings, that might be why - but you'll probably see the same thing on other sites.