This is because of a bug in Reason. Luckily it's very easy to solve and work around it. This error occurs when you save a song and another file with the same name in the same directory on systems that are set to hide the file extensions. For example, if you save a song called MySong.rns and it has a REX-file called MySong.rex, then Reason will try to load the Rex file as the song, resulting in a bad format message. For a quick fix - just rename the song and open it. For a permanent fix, do this:
1. Start the Windows Explorer
2. Go to View -> Folder Options
3. In the Folder options window, click the 'View' tab
4. In the 'Advanced Settings' field, disable the 'Hide file extensions for known file types' option.
5. Click OK

After doing this, Reason will probably report missing sounds when you open a song that you've had problems with earlier. Use the search and replace dialog to find the sounds and then save the song again. Doing this will fix the song.

This bug will be fixed in the next update.