Aboveground NM, the box behind the luminaire had a little water in the splices. Dried it out, sealed the back of the fixture to the building, problem solved. In another case, the UG feed had a little slice on one conductor in the UG part. Only tripped when the tide came in.
If your run is totally dry, try to isolate the problem by bypassing the UG feed with a temporary aboveground feed.
What type insulation is on the wire you used to feed the UG to the luminaire? THHN or THWN? or did you use UF?
Do you have a hi-pot? Some part of the circuit has a "slow leak".