Can't upload video to thread--Admin help needed

Every time I try and post a video to a thread I started, I get this message:

Your submission could not be processed because a security token was missing.

If this occurred unexpectedly, please inform the administrator and describe the action you performed before you received this error.

Any ideas what this means?

At this time you cannot do a direct upload of a video.

You can upload to You Tube and then post as a link.