Power issues in out-building sub panel - What's the best way to repair?
By having zero volts across the main it appears that 1 leg is open and the tools are back feeding so it appears there is 120v to ground on both.
the zero was a clue. With no load the 240 v can be there there is probably moisture in the pipe and no load provided a path much like phantom voltage there is voltage but no current.
No load and a path for voltage may be how the 240 is being measured with no load.
I have seen wires blow open in conduit in lumber and plywood mills more than I thought possible and the other conductors are usually damaged.
In some cases I will pull the conductor that is damaged and use it to get a location to dig, in others I use a scanner as long as there is not a reenforced concrete pad the scanner may work but having the damaged conductor also gets us close.
Depending on cable size I may drop a box at the location and then repair the conductors and pull the wire to repair the open leg.
At this point it is obvious the 70v leg is damaged but there may be insulation damage and even conductor damage on the other leg.
To tell at the feeder which one is damaged I would use an amp clamp and see which is loaded and the other may just show leakage.
If they are both pulling 10 amps let’s say change the load reduce the evens or odds the leakage usually remains somewhat constant but adding or reducing the load on the good conductor will make it easy to identify at the feeder breaker.
The conductor that is damaged should be disconnected at the feeder breaker until it can be repaired as you are paying for the leakage current and it will do more damage if not disconnected.