In /includes/functions.php in redirect() we have

	// Redirect via an HTML form for PITA webservers

	if (@preg_match('#Microsoft|WebSTAR|Xitami#', getenv('SERVER_SOFTWARE')))

As in Re: If your browser does not support meta redirection a user is running Webmatrix 3.0 (7.1.1932.0) on IIS Express 8.0.8418.0 and his SERVER_SOFTWARE environment variable is

Microsoft-IIS/8.0

However, he claims that he don't needs the HTML redirect - the server supports real redirects very well. phpBB should check more precisely in future versions.

I'm not familiar with IIS - from a quick view it seems the redirect issues have been cleared with version 7.5 and higher.