Hi,
We received a bonsai from Ikea as a gift and it's losing quite a lot of it's leaves.
First they get some small black dots and then they fall, even if they are still green.
You can see in photo "initial" how it looked when we brought it home and in the "after" photos how it looks now, including the black dots on the leaves.
I'm not an expert, so would very much appreciate if you know what's happening and what do I have to do to save it.
Thank you in advance!

Hi, and welcome here too
So, its not a ginseng but a "normal" ficus, but its still not a bonsai. This ficus is actually a number of trees brided together. So, same comment as in the dutch forum :treat as every other houseplant.
The problem is it being inside: our houses are way too dry, with not enough airflow or sunlight. You even placed it next to the heating, which makes the air even dryer... Ficus is known for dropping it leaves after being moved, so that quite normal. Find a good spot with lots of light, a constant temp and leave it there, it will probably recover.
Trees inside is always a problem ( there is no such thing as an inside tree ), so if temps go above lets say 15-20 again next spring place it outside, it will be much happier there

Thank you!

Make sure you don't have water sitting under the inner pot. Spots on leaves can be a sign of overwatering.
If those black spots where already on the plant when you got it, and your not getting any new spots, then that damage might have happend at the ikea nurcery. But if you still getting new black spots, then you need to find the cause ofcouse.