There's a bug in line 18 of templates\Traq2\roadmap.php, it's: <h2><a href="<?=$uri->anchor($project['slug'],'milestone-'.$milestone['milestone'])?>"><?=l('milestone')?>: <em><?=$milestone['milestone']?><?=(!empty($milestone['codename']) ? ' <small>"'.$milestone['codename'].'"</small>' : '')?></em></a></h2>
and the bug is that the anchor is created in the form of 'milestone-'.$milestone['milestone'] and it should be 'milestone-'.$milestone['slug']
Also there's no error if I go to a nonexistent milestone like milestone-qwerty, it's just an empty milestone page.
Thanks for reporting this. I've gone ahead and fixed this.