Another vote for it being installed backwards.
The ball-side fitting should run to a dedicated manifold pressure source (or as close to a dedicated source as possible). That way, when the manifold is pressurized, the pressure runs down the line, presses the ball up and compresses the spring, and allows the pressure to reach the wastegate/wastegate actuator so it can regulate the pressure.
If it's run backwards, the manifold pressure will never reach the wastegate, and an overboost situation will occur.
Hope this helps.