Electrical Probs with QingQi 200...

[forchetto]

I should take care of a multi-wire connector that resides above and to the right of the battery, next to the chassis, one that has a rubber holder that fastens to a small bracket by pushing it. My friend told me that I will most certainly find it installed in such a way that it would allow to get water in

The item you mention that is held by a chassis tab is the winker relay. There's a bunch of wires with a connector further up, but is well protected from water thrown by the wheel.
The thing to check is the tightness of starter relay connection posts and specially the main engine earth cable. This cable is responsible for all current out of the generator and all current into the starter motor. Any resistance there will make things funny.
The following pictures show this cable, at the chassis end and at the engine end. The main battery earth cable also has to be tight.