Hm, right. It really seems like it cannot deal with a multivariable problem ... Here is a simpler example that produces the same error message:
FindRoot[{x == Quantity[1, "Seconds"],
y == Quantity[2, "Meters"]}, {{x, Quantity[3, "Seconds"]}, {y,
Quantity[4, "Meters"]}}]
(* FindRoot::nlu: -- Message text not found --
({3.s,4.m}) ({x,y}) ({2.s,2.m}) ({2}) (Removed[$$Failure]) *)