Open up your forumdisplay template and copy & paste the code here.
<html>
<head>
<title>{$mybb->settings['bbname']} - {$foruminfo['name']} </title>
{$headerinclude}
{$rssdiscovery}
</head>
<body>
{$header}
{$moderatedby}
{$usersbrowsing}<br /><br />
{$rules}
{$subforums}
{$threadslist}
{$footer}
</body>
</html>
Do you have any plugins which add title/meta information?
Yes I have installed Dynamic Metas by CrazyCat ...
Try deactivating it and see if it helps.