I have just done a fresh install of ISPConfig on Ubuntu 18.04. When I create new websites through the control panel Let's Encrypt seems to work fine, I can access the new websites through https without any errors or problems. Which makes me think apache is configured correctly. But for the Control Panel itself it doesn't work. I cannot access it through http and through https I get the not secure icon and if I click that, it says the certificate is invalid. I did create the certificate when I installed following the "Perfect Server" guide on here. I have tried reconfiguring and create a new certificate but that didn't help.