I really dont like this solution, but it should work for you as a dirty patch.

in /libs/ubbthreads.inc.php

FIND:
Code
return "$http://{$_SERVER['HTTP_HOST']}{$url}";

REPLACE WITH:
Code
return "//{$_SERVER['HTTP_HOST']}{$url}";

Protocol-relative links (PRL), also known as protocol-relative URLs (PRURL), are URLs that have no protocol specified. For example, //example.com will use the protocol of the current page, either HTTP or HTTPS

Last edited by isaac; 07/23/2019 9:06 AM. Reason: added scheme def
current developer of UBB.threads php forum software
current release: UBB.threads 8.0.1 // wip: UBB.threads 8.1.0
isaac @ id242.com // my forum @ CelicaHobby.com