I also get this problem a few times a year for no reason that I can find. The last time I received this message from Tech Support:
<<Some specific video files might face problems during playback unfortunately due to occasional encoder problems that Express Scribe is unable to resolve. If you have a few of these videos, an easy troubleshooting is usually to convert the videos to reencode them. You may use the Prism software trial download for the purpose and continue with Express Scribe again.>>
I am not particularly tech literate, and I don't know anything about what to do with my client's files or their encoding, so I'm left stumped. I've tried changing the file extension but it doesn't make any difference. Would be nice to have an explanation for non-computer experts.