I'm going to go out on a limb here. I don't think the backups between the two flavors of BaseCamp are compatible. What does work is to export the folder containing all of your data and then import it into the Windows app. You'll get the data, but loose all of your organizational formatting, sub folder and such. As well, I've had much better results using Garmin .gdb files for exchanging data between Garmin apps than .gpx. Garmin is doing something weird with certain gpx files that render them incompatible between applications.