Hmm. sprayed yesterday, but there are more spots today. Active ingredient is fenpopatrin. The spots did start to appear after I moved the plants to a new location, an improvised greenhouse.
The greenhouse is pretty makeshift, so it is possible that water dripped onto the leaves, or that sunlight passing through water (accumulated on the roof) somehow made a hotspot on the leaves.
The thing is that it seems to be spreading, kind of. There is a similar spot on a plant appx 1.5 meters away, separated by two other plants. I have already removed the effected leaves from the photographed plant. Guess I can wait a day or two, see what happens with the other plants.
Thanks for the input.