Strange Characters / Character encoding problem with hashtag

I'm getting the dreaded "� " character when someone uses a special character in a hashtag.

At the moment, you can see this problem on http://clients.gertowebservices.com/wfs/node/1 - I also attached a screenshot.

In this case a Tweet mentions "Armenië" (no hashtag) , which is displayed correctly, and another with hashtag "#Armenië" , becomes "#Armeni�"

If I remove the following code, the characters display normally again (but hashtags do not have links any longer, obviously) so I do not think this is a character encoding problem with my site:
twitter_block.module, line 287:

  // Linkify tags.
  $status_text = preg_replace(
    '/(^|\s)#([\wåäöÅÄÖ]+)/',
    '\1#<a href="http://search.twitter.com/search?q=%23\2">\2</a>',
    $status_text
  );

I sadly don't know enough about regular expressions to find the problem here. Thanks in advance.