Symptom: When click on the URL link in the UCP welcome email the website is not reachable "This site can't be reached" (see screenshot below)
Issue: the UCP module in FreePBX likely has been updated hence overwriting the custom URL in the "Ucp.class.php" file. This will result that the links in the UCP email are http and not https
Solution: Execute the steps under topic: AMPWEBADDRESS in this article.
If needed after solving resent the UCP email containing the correct https URL's to the respective customer