I tried it on an Android phone and tablet both, and neither would open, including with the full "Index.php" added. But, oddly enough, when I tried on the tablet to open one of the forum pages by full URL, it did work, but gave a blank page when I went to the home page. I was not logged in then, so I then logged in, and the home page opened fine. It's a little odd for sure. My laptop (Win 10, Firefox) has had no problem, but I'm permanently logged in on that one. I'll check Ubuntu later.
e.t.a. definitely a little odd. I tried it on another machine, on which I was also logged in, and as soon as I logged out, the home page went blank, with no option to log back in. So I returned to this laptop, where this message appeared, with a little note saying "logged," but no option to edit or reply until I logged in again.