I've installed ISPConfig on a subdomain ispconfig [dot] domain [dot] com, the installation was successful and i can enter to ispconfig panel under https :// ispconfig [dot] domain [dot] com:8080 Now i'm using the domain (domain.com) for a website, and i couldn't have SSL installed on it When i tried to create a let's encrypt SSL certificate for the domain domain.com, I got this error on Firefox and the Code: Websites prove their identity via certificates. Firefox does not trust this site because it uses a certificate that is not valid for domain.com:8080. The certificate is only valid for ispconfig [dot] domain [dot] com.
That's not related to each other, it's perfectly fine to use domain.com for a website while using a subdomain of that domain for ISPConfig UI. Have a look here to find out why no LE cert could be issued for the website: https://forum.howtoforge.com/threads/lets-encrypt-error-faq.74179/
No. Like Till wrote, follow instructions in Lets-Encrypt-Error-FAQ to find out why certificate was not created.