Error: Unity3d: Failed to load package manager

To solve you have to:

• Assets -> Reimport all
• Restart Unity

If not working:

• Delete Unity using AppCleaner or similar.
• Install using this guide, You can not skip any steps, especially those that are in bold.

GO->Applications
double-click the Unity folder
right-click Unity, and select Open Package Contents
double-click contents
double-click MacOS
right-click Unity and choose Copy "Unity"
right-click somewhere on your desktop and choose Paste Item. (this is your backup)within Finder, right-click again on Unity and choose Move to Trash
drag the patched Unity from the zip-file into the MacOS folder
Now the *.ulf file
GO->Computer
double-click Library
double-click Application Support
double-click Unity
right-click on the *.ulf file and choose Copy "Unity_v4.x.ulf" or "Unity_v5.x.ulf"
right-click somewhere on your desktop and choose Paste Item. (this is your backup)
within Finder, right-click again on *.ulf file and choose Move to Trash
drag the patched *.ulf from the zip-file into that folder[