I've mentioned this to someone else that had it set like this, but the board URL really should be a full URL with protocol. The board URL doesn't dictate how people can access the site. It is primarily used for email -- though it is actually used for Facebook stuff (because you need to tell FB a particular URL that you're on).
You're running Apache, but this appears to be specific to your web server, as it's actually redirecting with just this URL:
Whereas other servers handle that without an issue. (Note that now that I'm trying it again, I'm not reproducing it directly, but I do still via FB.
The redirect isn't being output by XenForo, as it specifically mentions that it's charset ISO-8859-1 (whereas XF is 100% UTF-8). I'm not sure if you have any custom redirects anywhere that could be triggering this.