Assuming this is a fungus, I sprayed the leaves with baking soda water, but it did not help.
Right now, the tree is on a windowsill, getting plenty of sunlight. It is also gently watered twice every week or so.
It is still in the pot and soil that I got along with the bonsai at the nursery.
My previous bonsais have had this problem, but it somehow just went away. But this one I'm a bit concerned because half of all the leaves dropped already.
Any help is really appreciated!