Car has hesitations when accelerating

I think you need to stick with what you found on the actuator. Even though we don't have a manual on your vehicle as it is not offered here in the USA so I can't give you specifics on your wastegate, I can speak about how these work in general.

Basically the wastegate is required to regulate boost pressure and in most all cases where there is an under boost condition it is because the wastegate is sticking allowing the boost to escape. There are clearly other causes of this especially on diesel application when you introduce soot. However, the wastegate sticking open should be the first place to start.

If you have a scan tool, are you able to monitor live data? You should have a data point called boost set point or desired boost, or commanded boost, or target boost, or something similar to this.

Then you should have another data point called actual boost position, boost output, boost %, or something indicating what the boost sensor is telling the PCM what the output is. More then likely these will not be the same which indicates you have a leak and it is more then likely the actuator.

Even if you don't, have a scan tool then just finding that there is a sticking spot then that is most likely the issue. The spring is probably not able to overcome this spot and so when you are accelerating it is hanging open rather then being closed. If you look at the second picture below that will show when boost is applied to it. The arm pulls the diagram (red) and compresses the spring (green). This releases or regulates the boost to a certain PSI. When this sticks and the spring cannot push the diagram back to its resting position then the boost will only build to a certain point.

Does this make sense?

All of the above could be the issue. I doubt that the humidity is lubricating the shaft but the ground/wiring issue could be the issue. The odd thing is why it is working when it is wet out versus it doesn't. That is normally the case when you have a wiring related issue that it acts up when it is wet out.

So if it were me since you found it sticking, I would replace the wastegate first and go from there. Normally the simplest answer is the correct answer. It is less likely to be odd issues like that.