Get lost 'File Manager' path
I always find myself forgeting to save the "File Manager" path found in Tools/Configuration/File Manager/File Manager/File Manager path.
To find it go to Reports/System Info, look up phpInfo() and click on View.
Now look for _SERVER["DOCUMENT_ROOT"]. Use the value in the right as your "File Manager" path.
You should also update the "File base" path found in Tools/Configuration/Interface & Features.