SSL issue - This site can’t provide a secure connection

This issue started recently. All sites on my virtualmin server are affected but non secure http works ok as does sending and receiving email.

I can’t access any of them via https - the message “Cannot securely connect to this page” varies from browser to browser. I can, however, access the admin backend by adding port 10000 or 20000 via Chrome which then tells me that the site is secured with Let’s Encrypt.

For the last 9 months I’ve had my own certificate on the system but switched to Let’s Encrypt AFTER this issue started as I though that may have been the problem.

I’ve tried multiple devices / browsers and get the same result.

I’m running Ubuntu 18.04. Any help would be much appreciated. My knowledge here is pretty basic!