Electrical – Line-to-Line Voltage Reads 0V (or low, 15V at most) on Primary of Main Breaker (L-N both read ~120V)

My neighbor has a hot water tank that quit working earlier this week. I checked the breaker – looked good. I measured the voltage at the hot water tank; it read ~15Vac. I checked the double breaker for the hot water tank and it also measured ~15Vac. One step further, the line-to-line voltage coming into the main breaker read ~15Vac. Both 120Vac L-N branches give a voltage but one is at ~118Vac (good) and the other is around ~105Vac (hence my ~15Vac differential, it appears on the surface). When I open the main breaker, both L-N at the main breaker primary read ~120Vac. The hot water tank is the only 208/220 circuit in the house (and the only double breaker). Any idea what could be causing this suddenly? It looked to me like both branches were being fed, in phase, and that is why the difference between them is literally the difference between them. I had read that an open circuit on one of the main lines could cause this (the open line being backfed by the good line through the double breaker). But I'm not sure why the hot water tank would have worked up until this point.