[Zope] Zope Management Interface: Bug of IE?? or Zope Feature

Hello, in a hand-made ZClass the tabs of 2 of 4 instances show this-->>>> <a href="manage_UndoForm" ----->>>>target="blank" > <span style="color: #000000;"><strong>Undo</strong></span></a> This means the manage - view opens in a new window! I can't reproduce my behaviour to find a solution (the Classes have got an attribute target but can this mix up Zope??). I use IE 6 (but 4.7 shows the same) on Win 2000 server. Thanks!