I ran into an interesting situation that I didn't realize. If you have a filename with a period in it, then FMP13 can't open it if hosted on FMS14. For example a file named "data.something.fmp12". An FMP14 can open it. And an FMP13 can open it on an FMS13 server. It is only when you have FMP13 opening a file hosted by FMS14 that it is a problem. Remove the period and replace it with an underscore or space and all is good such as "data_something.fmp12".
So just be warned to watch out for periods if you want backwards compatibility with your FMS 14 server for earlier version FMPs.