I created a project with version 15 on my Dell desktop. Someone else edited the project on his MacBook Pro also with Premier Elements version 15. Now when I try to open the project this is what I get. The media never renders. This has happened with all the projects edited on the Mac. The project do not have more than 3 minutes of video.
Premiers Elements is not a multiuser program. And, there are differences between the Apple and Windows versions.
I don't think that there was ever an intent to make them cross compatible. On top of that, there are a lot of preview and cache files generated in the process of editing a project. It would not be easy to figure out how make those work in a shared environment even if you tried to make it multiuser.
We have figured out how to solve it. The images were not loading. By going into Expert mode and viewing the Project Assets, we can see the assets that have not loaded. Clicking on the Asset, then makes it load and all is fine.