Thanks for the idea but this did not cause the issue apparently since changing it back won't help.
So any other ideas why this is happening?
Maybe an interesting observation - when browsing such a "full-width" page with Firefox I can see that the module in the center pane is covering the entire screen when the page loads but once loading is completed it snaps back to a size where it seems that the left and right borders are padded. As if there is table space reseved for the empty panes.
Best regards,
Tillman