As of 1st Oct. 2021 one of the root certificate used by “LetsEncrypt” as used on this website expires.
What that means is that some old web browsers may not work with the modern encryption system used on this website.
More information can be found here —
https://letsencrypt.org/docs/dst-root-ca-x3-expiration-september-2021/
If you have any problems with an old web browser then the best thing to do is to update your browser. Which is free to do.
Mike