Albrecht--
Thanks for the clarification; you're doing right--as several of us have said, you do seem to know your way around. I don't know the C-Lux 2; if the minimum ISO limit is 400, you're doing exactly the right thing.
(There's one thing to consider in that regard, though: if the camera is choosing an ISO with a top limit of ISO 400, in low light that will force a lower shutter speed than a higher ISO would. So even though you're getting a better image structure in terms of 'grain,' the lower shutter speed may tend to offset that.)
You say the focus on on the ventilator (or "fan" in US English). But if you're using AF that's just what you were pointing the focus point at; focus is still only approximate. You don't necessarily know what the camera decided to focus on.
Of course, in downsizing the image for forum posting, we lose some of the data that we need to evaluate the problem.

But the watch looks fairly sharp to me. Maybe the AF chose a more general, overall focus.
This picture may be a little disappointing, but given that it was shot indoors, I think it may not be outside the expected range.
I'd like to see an outdoor shot, as you said. And since we're checking the camera's performance, in order to eliminate any outside influences, you'd want to use a tripod both indoors and outdoors if you can.