i've found (hope so) for you ...
if you have activated the "Verwaltung des Blog"-plugin you see a link to the cp ... if you follow this link the browser goes to "index.php?/admin" ... so ... if you then click for example on the "Statistics"-plugin (i think this is the only link with this behaviour) the browser goes to:
index.php?serendipity[adminModule]=event_display&serendipity[adminAction]=statistics
and this won't work - welcome back to the index page
0.8alpha12