I get the attached error message when I try to open the Assets starter solution made in FMPA v13, in FMPA v12.  

 

At first I assumed that this meant that there was a general lack of forward compatibility between files made in v13 and the v12 app.  However, I don't get this error with other starter solutions, or files made from scratch in v13.

 

Also I can open .FMP12 files created in v12 in v13, add v13-specific features (like Pop-overs) and am still able to open them in FMP v12 (though the pop-over button does not show). 

 

Does any one know what makes certain v13 files incompatible with the v12 app?  

 

 

Look in File Options

You can define which version has to be used

I remember when version 10 or 11 (don't remember!) came out and I started implementing features not available in earlier versions, I , after making many important changes that would have made a solution just ugly in previous versions, disabled opening the file in any version less than 10 or 11. Perhaps this particular starter solution has done the same.

Yep.  You all are correct!  Thanks.