Hello Rob,

It seems to be some configuration level issue or may be due to the SSL renewal done recently.

Check this reference link and see if that helps.

Also try to disable all the plugins once and activate default theme to see if there no theme or plugin level conflict.

Thanks.

Hi!
Check .htaccess file, if it is correct or not:

htaccess

If you have some security, ‘pretty links’ or other plugins which can be responsible for redirects, try to switch them off as well.

Keep in mind that your browser can save these redirects orders and better check your site by opening each time incognito.

A plugin can be disabled by renaming its folder:

Managing Plugins

Try to enable debug log as well:

Debugging in WordPress

How you found it? Your certificate looks fine.

If you have a backup, try to restore the site from it to see if it will solve the problem. Each time you need to open your site in incognito and close after because the browser will be remembering this redirect.

You can try the manual update:

Updating WordPress

Disable all plugins and open the site:

FAQ Troubleshooting

Try manually update your active theme.

Using Themes

If it will not help, rename the theme folder. If your active theme will be not found, the site will show an error or blank page but you will be able to access admin if the redirect is due to the theme. In this case, go to ‘Appearance > Themes’ and WordPress will switch to one od default themes and you will be able to install another one.

And for additional information:

FAQ My site was hacked

I see no problems with the certificate in Chrome and in Firefox, certificates are checked by OS, so, you can find and update it in the system or try your site from another device.
But sometimes these caching problems can be difficult to find and solve…

Let’s Encrypt certificates works fine and widely used, so, I don’t think there is something to worry about. You can take another site with such certificate to compare )