I think its the same issue that technota has:
I have a mutlisite installation running buddypress and bbpress. In buddypress forum option off (so no conflict with bbpress) Working fine, but when Classifieds plugin activated the permalinks for the forum doesnt work no more. (page not found) Also created a custom menu (not success) re-activate bbpress and renewed custom menu (again no success). Permalinks set to day.
Note: when remaking a custom menu, the permalink for the forum changed into post slug with date and after remaking menu's changed to same permalink with date and number (number increases by remaking menu: website/2012/01/26/menu-1, menu-2, etc) When de-activate Classifieds, the permalinks for the forum changes back to normal state: website/forum/topic.
Also I havent change any settings in classifieds when activated.