I contacted Celemony support for the same reason (Logic Pro X freezing upon adding an instance of Melodyne 4 plug in - spinning pizza wheel of death).
Here are excerpts from their response as of 2/8/2016:
With the latest Logic release we have indeed a problem with the new Melodyne version.
We have other user reports, that say that the beach ball appears if they are using the library, instrument or channel strip tabs. If they are not using them, Melodyne seem to work without issues.
Openning older projects, that contain Melodyne and let Logic crash on opening:
You can isolate issues with audio devices by starting Logic Pro X with no audio driver. Press and hold the Control key immediately after Logic Pro X starts to open. A dialog asking whether you wish to launch audio drivers will appear. Click "No" and see if Logic Pro X starts up normally.
Source: https://support.apple.com/en-us/HT200260
After this procedure you can close all instrument, library or channel strip panes, save the song with a new name and open it again. As the problem appears only if the panes are open, this newly created song should at least work, as long as you don’t open any of the panes mentioned above.
I haven't had time to test this out myself, but I hope this helps.
Personally, I'm just waiting for the latest patch to remedy the problem.