I have to second electures above comments and those of Lostazhell, even with an delta service (or open delta) "high leg" there would be no way to get 255 volts as line voltage, an open neutral does not explain it.
There has to be a ballast/transformer/step-up influence of some kind at work here. The four that work are normal incandescent par 38 lamps? What is the voltage at thier lamp socket? what do the fixture nameplates if any say? I need more info to attempt a diagnoses but find the 255 volt report revealing in that there simply has to be a step-up process of some type involved. Let us know what you find....
