Slitherine

A friend of mine and myself have bought the CNAW to play PBEM between us. Nevertheless, when we sent the fisrt turn to the another, the following message appears: "Exception, the file could not be loaded". I can see the file (so I have saved it in the correct place), but I cannot open it. We have tried it several times and changing the order (sometimes I start the game and send the file and sometimes my friend send me the initial turn).

Has anyone had the same problem? Does anyone know how to fix it?
Thanks