I have the same problem as you with Windows 7 and with any kind of video file.
To solve I see that the twonky media server in the status of the server, in the videos section always give the name of some folders when it makes the hash. I decided to remove from the shared folder and then goes I made to rebuild the video shared folders and now goes well.
@yippikay thank you for your answer. However I did not fully understand it. Do I need to use a different folder than the shared folder? Please rephrase your answer. Thank you.