All cplanes seem to have disappeared in existing files - I opened 4. If I open the file in Windows Rhino the existing cplanes are there.
I think this is the same problem that caused this problem: Minor bug- blank layers and osnap panel when launching file. Try the workaround mentioned in that link and see if that fixes it.
Command-N (open new file) did not work for me. I do not see my CPanes in View-Set CPlane either.
I rarely use the CPlane panel but I see the same problem in the post above as well. I solved blank panels by switching between open models,
Tried creating a new file and a new CPlane. The CPlane showed in the menu and CPlane panel. Saved the file, quit Rhino and reopened. CPlane was gone.
Reverted to 470. CPlanes are back!
I see this, and is not exactly the same cause as the Layers panel problem. This will be fixed in the next WIP release.