That error usually means that the video is currently being processed due to an edit or sound edit you made, and you will be able to resume editing after it has finished processing. This error often occurs when you hit save and then you try and add an edit you forgot. Until the process has finished you will just see an error with bunch of Youtube video icons stacked up on each other. I often see this kind of error when adding music tracks that Youtube allows people to add. These music tracks pay for themselves through Youtube Ad money which is how you can legally add them as sound tracks.
Another message you may also end up seeing is “Sit tight! Your video edits are about 90% done. Come back to this page in a little while.”