cant delete a session

[Moriaty]

i cannot delete any session. when i try to delete a session, PO5 starts loading and the pc turns really slow for about 20 minutes. after that, PO5 freezes and when I try to end it by ctrl+alt+del, it lasts about 10 seconds and then I get a java-error message. After that, PO5 is closed. When I restart it, the session still is not deleted.