…It only seems to happen when the omnibounce is on and pointed straight on instead of "bounced" at 45 degree...
I get the same lens flare on my Pro1 when I use a slave flash near to the field of vision of the lens. The lens hood helps a lot.
Perhaps the 420EX+omnibounce protrude too much to the front and your lens can “see” the flash. Try screwing a lens hood in the adapter or blocking the light from the omnibounce to the lens with a piece of cardboard and test for flare with and without the hood/cardboard.
rpolitsr's advice is excellent.
