Then the reason is still the fact that you don't have a pane called "ContentPane"
If you add a module to the leftpane and change the skin, to a skin without a leftpane, DNN moves the modules to the contentpane and that doesn't exist...
So DNN does work without a pane called "ContentPane" but it crashes if you change the skin and the original pane for a module does not exist in the new skin.
(I guess you figured that out already, this is more for people that read this post later)