Pubcookie problems with Drupal redirector

Let's describe a situation. Drupal is installed in the webserver root, and you want to guard a subdirectory on the server, which is completely separate from Drupal, by Pubcookie authentication. The problem is that Pubcookie internally uses another URL on your server named /Pubcookie.reply, and this URL gets intercepted by Drupal redirector. You can get various sorts of errors like "No granting reply". Pubcookie.reply is a magic URL that Apache knows how to reply to, but Drupal was rewriting it to index.php?q=Pubcookie.reply or alike. As a solution, add the following rewrite rule:

RewriteCond %{REQUEST_URI} !=/PubCookie.reply