The Mx9 Beta can not be downloaded anymore. Did you try to update it with the new core atlas?
Regards,
Ronald
[EDIT]
Check also this out: https://docs.mendix.com/refguide/moving-from-atlas-2-to-3#1-introduction
I added the design properties to the jason file from an older project. The spacing top ec is working again. But my custom design properties keep running into erros as wel. No idea why, I'm no programmer but they were fine before. had to delete them an manually add the classes back to the elements.
Very annoying.
Same issue here