Usually this happens when you try to play a Cloud video that has not been played in a long time or this could be a video in the Cloud that is not supported anymore. Either ways, the solution is as follows:
1. Download the video that is not playing in the Cloud.
2. Use RealConverter and convert this file to MP4 format. (Even if it is currently in the MP4 format)
3. Re-upload the file to the Cloud and try playing it.
Please let us know how it goes.