I see that you are on UBB.threads 7.6.1. A lot has been fixed since then. Here is the fix for your issue affecting version 7.6.1

DESCRIPTION:
This fix returns protocol-relative URLs since an overzealous ModSecurity firewall rule may not like having URL query strings with a suggested HTTP/HTTPS protocol in them.

in /libs/ubbthreads.inc.php
AROUND LINE 985, FIND
Code
	return "$http://{$_SERVER['HTTP_HOST']}{$url}";

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