Damp from floor / wall?

Hi,
we have a problem with a mouldy smell and a bit of mould. In our living room, along the outside wall, there is a gap between the floor and the wall. Not sure what's in the gap, but it is sandy. The floor is a reddish concrete? It's a terraced house (build mid 1950s) and the outside wall is a "roofed" passage to the back garden, i.e. there is no rain or alike getting there.

Along the wall, it seems a bit damp, and for a few inch on the floor, a bit of mould can be seen. The carpet and undercarpet smell mouldy. A bit further into the room everything is fine. The area of the wall is mostly "covered" by means of a couch, furniture, etc. We don't have skirting boards at the moment, i.e. the carpet is just lying on the concrete floor, and this gap is not sealed or alike.

I'm not sure whether this could just be condensing humidity, because the wall is a bit colder, but it seems a bit unlikely in the current warm weather. Or whether there is damp coming through the wall/floor/gap between floor and wall?

Before we moved in, I "painted" the floor with undiluted SBR, mainly because it was a bit smelly. Can this gap be sealed somehow?

On the pictures, only the tiny dots are mould/mildew or alike, there is also white paint on the floor...

Best wishes,
Andre
(click on the image for larger size)