Same thing with my newly repotted Paph venustum. I cut the leaf that had the brown spot (actually half the leaf but in the middle part). Then flushed the whole pot with water, about three times. The leaf with the 'burn' kept dying after I had cut it back to the green bit, but at least now, none of the other leaves are showing the same signs... I can only put it to a new fertiliser which I got becoz I was told at the hydroponics shop that it was good for growing plants by hydroponics.
Good luck with the phrag.. hopefully it gets through it...