The blender system is structured with Area's 1 and Regions 2.
The application window is always a rectangle on your desktop. It is divided up into a number of re-sizable areas. An area contains the workspace for a particular type of editor, like a 3D View Editor, or an Outliner.
An area is divided into regions (a sort of placeholders). For example:
- Tool Shelf
Scripts can use these regions to (for example) add custom panels to a region.
Now the Error
Error, region type missing in - name is send to the console when a script wants to access a region in a area for temporary windows.
This used to happen when users open a user-prefs or render window (those are temporary screen and). And then start using it as a regular window (split it, change space types... etc). This was solved in 2015, but could still be possible when scripting.