My thoughts are it’s not a CS5 file or it’s corrupt.
Are you sure it’s a CS5 file?
Since you only have CS5, your computer doesn't know about newer versions and thinks everything is CS5. The shared content plugin erro is a dead giveaway that it's newer.
OK, Peter, that makes sense. I will ask the sender to back-save it if possible. Would that be in the File>Export area?