I have been successfully (and very happily) developing for Unity 5.5 on a Mac until this issue.
After switching to the Dec.19th EAP, the two plugins I had been using with the previous EAP don't appear to be working.
I was using the Unity and Heap plugins.
No other plugins are installed.
I've tried all the usual stuff. - uninstall/reinstall plugins - checked for latest versions etc. - I even read the release notes