Video Upload Fails - Security Token Missing

[emailapphost]

I try to upload a video file in, as an attachment, in a post and get this error message.
What do I need to do? Thanks.
I want the locally hosted video file to play in the post, is there a better way to do this than to upload it as an attachment?

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.