A bad neutral couldn't get 255V from a 120/208V system. At most it could be 208V
As a matter of fact, the situation you're describing is impossible electrically without some other influence.
255 Volts is a reading typical of a metal halide ballast's open circuit voltage.
Have you taken a mirror and a flashlight and looked through the hole for a ballast possibly mounted remotely? Some of them can be a real nightmare and take a real long arm to replace.
Taken a L-N voltage reading at the fixture's junction box line terminals instead of the lampholder? With a solenoid type tester?
