Is your Aperture library on an external drive or on your system drive?
Are the videos imported into the library ore referenced? What kind of videos, what is the camera type?
Have you tried to recreate the thumbnails for your movies? You can do that by selecting the movies and using the command "File > Generate Thumbnail".
Or try to set the poster frame again. The movie poster frame will show as thumbnail in the browser. The poster frame can be set in the Movie Editor. Click the cogwheel, when the movie controls are displayed:
I went to iPhoto 11 (version 9.4.2) and switched library's importing the Aperture Library. Same thing. All movies have blank thumbnails which function.
Exported a movie to the desktop. A generic xxx.mov icon appears with the proper title but no thumbnail image.
Opened iMovie. Imported the movie. A blue thumbnail was present. iMovie asked me to create thumbnails for Aperture and iPhoto movies. Accepted. in iMovie the blue thumbnail was gone away.
I reopened both Aperture and iPhoto. Same blank thumbnail.