I am using wordpress 3.3.1 with PageLine's WhiteHousePro theme, which has a built in function for choosing "featured" thumbnails for individual posts.
After updating events manager, the widget within the post editor that was resposnsible for this feature disappeared. The problem persists with no other plugins active and the events manager settings reset.
The events-manager option "Disable thumbnails?" has no effect on this or on how my posts are displayed.