Editor/UnityEditor.UI.dll Timestamps/assets maps out of sync.

DON'T JUMP THE GUN, THIS IS not a DUPLICATE!

Unity 5.01 WIN7 - I know how to fix this: Timestamps (###) and assets (###) maps out of sync. for files in the assets hierarchy:

• identify missing files, move them from their location to a newly created tmp folder under the same hierarchy

• move files right back to the original correct location, Unity detects and syncs up automatically,shows all files as normal.

• delete all 'tmp' folders

• at worst (re)import all assets... but I avoid that

But HOW TO FIX THIS?: Timestamps (###) and assets (###) maps out of sync.

Asset 'C:/Program Files/Unity501/Editor/Data/UnityExtensions/Unity/GUISystem/Editor/UnityEditor.UI.dll' is in timestamps but is not known in guidmapper...

Asset 'C:/Program Files/Unity501/Editor/Data/UnityExtensions/Unity/GUISystem/UnityEngine.UI.dll' is in timestamps but is not known in guidmapper...