This problem has been around at least two years since we started using Encore CS3.
Create an Encore BluRay Project on a Mac. In our case, a Mac Pro Octo 3.2 machine running the latest OS, Quicktime and Encore CS4.
Save that project.
Open the project on a second Mac that contains the BluRay burner. In our case a Mac Pro Quad 3.0 machine running the latest OS, Quicktime and Encore CS4.
Get the error message, "This Project was created in Windows and cannot be opened."
I posted a query in the Cow Adobe Encore forum, but no responses so far. I'll let you know if I get anything.
Haven't called Adobe Tech Support because the last time we tried that, they could not figure out why I didn't have any Windows Machines in the shop so how could the project possibly have been created on a Mac? They're not very helpful in my experience.
So I'm just letting you all know, Encore still does not have anything to make you "Come for the BluRay, Stay for the Flash." Fortunately we do have NetBlender's DoStudio in here for real BluRay projects and it works flawlessly.