I would suggest starting with cleaning the connector blades in the plug that goes from the left side of your electrical panel here, over to the engine. Also, if the bike has been sitting quite a while, the voltage regulator's contacts will probably be a little bit crusty. A points file or some very fine sandpaper will clean them in just a few strokes: they are silver, so they are very soft. Don't sand them too long, or you will wear them down enough to require readjustment of the voltage regulator's set-screw.
It is very common for the contacts in the plugs and the regulator to corrode slightly with lack of use. I always apply a drop of LPS-1 to them, although I am not sure if you can get that in Europe? It is a form of contact cleaner-protector, and is very unique (don't try LPS-2 or LPS-2.5, those are for different things and are more like WD40 penetrating oil).
Do you have an ohmmeter? You can check the field coil of the alternator with that. Post back if you do, and either Bryan or I can walk you through that test.