and suddenly the editor spits out errors (855411)

Unity Engine
1

Just happened, no idea why.
Starting with this one

error CS1704: An assembly with the same name `UnityEngine.UI' has already been imported. Consider removing one of the references or sign the assembly
/Applications/Unity 5.5 navmesh/Unity navmesh.app/Contents/UnityExtensions/Unity/GUISystem/UnityEngine.UI.dll (Location of the symbol related to previous error)
/Applications/Unity 5.5 navmesh/Unity navmesh.app/Contents/UnityExtensions/Unity/GUISystem/Standalone/UnityEngine.UI.dll (Location of the symbol related to previous error)
error CS1704: An assembly with the same name `UnityEngine.Networking' has already been imported. Consider removing one of the references or sign the assembly
/Applications/Unity 5.5 navmesh/Unity navmesh.app/Contents/UnityExtensions/Unity/Networking/UnityEngine.Networking.dll (Location of the symbol related to previous error)
/Applications/Unity 5.5 navmesh/Unity navmesh.app/Contents/UnityExtensions/Unity/Networking/Standalone/UnityEngine.Networking.dll (Location of the symbol related to previous error)
error CS1704: An assembly with the same name `UnityEngine.HoloLens' has already been imported. Consider removing one of the references or sign the assembly
/Applications/Unity 5.5 navmesh/Unity navmesh.app/Contents/UnityExtensions/Unity/UnityHoloLens/Runtime/UnityEngine.HoloLens.dll (Location of the symbol related to previous error)
/Applications/Unity 5.5 navmesh/Unity navmesh.app/Contents/UnityExtensions/Unity/UnityHoloLens/RuntimeEditor/UnityEngine.HoloLens.dll (Location of the symbol related to previous error)

Compilation failed: 3 error(s), 0 warnings
2
  1. Close Unity
  2. Delete the Library, Temp, and obj directories
  3. Reopen Unity, it’ll automatically rebuild scripts and that error should go away