Go to your user preferences (CTRL+ALT-U) and check under input that the keybinding is set correctly via: screen > screen editing
Here is a screenshot of what it should look like (Ignore Theme, and I expanded the boxes):
If these are set up correctly (all fields have their proper values) then try to load factory settings option in the file drop-down menu then attempt to open the file and see if the problem persists.
It is possible that this is a bug but I never ran into this problem before.
As a side note if you mouse over the edge of a screen and right click you will be presented with two options: Split Area & Join Area
Also I have heard that many people have problems with blender not properly refreshing and that minimizing the window then bringing it back will fix the problem.