Potential Ground Fault Issue? Diagnosis?

I built a free standing garage with main power tapped from the home breaker box.  The contractor pulled 4 wires via a conduit to the freestanding garage panel which supplies lights, a garage door, and two 20 amp outlets.



Everything worked well initially, but about 1 week ago one section of the lights in the building went out.  The contractor came out and found that one of the two hot wires in the garage panel was only seeing 100 volts and that went down if anything else was turned on.  He swapped one of the ground wires for the bad hot wire and everything now works. 



What would cause a situation like this where the hot wire apparently had a voltage loss?  Is it safe to continue with the fix where the faulty wire is now the ground?



Thanks in advance!
      


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