LN-MAG, in order to try helping you with this problem I´m posting an image with the main/most usual problems that might occur to a tree´s leaves. The picture itself is written in Spanish, but I´m posting its translation here for you to understand it. Oh, the first 2 cases correspond to new leaves growth and the next 6 examples correspond to older or established leaves. (That is what the black line separates)

Hope this helps,

#1 Severe Nitrogen deficiency.
Small new leaves presenting a white/yellow coloration.

#2 Calcium deficiency
New pale and wrinkled growths (It could also be an overdose of potassium, magnesium)

#3 Iron Deficiency
Greenish nerves and old yellowish leaves

#4 Phosphate Deficiency (Phosphorus)
Older leaves presenting a yellow color with dark spots. The leaves die and fall rapidly (Note the similarity to early nitrogen deficiency)

#5 Magnesium Deficiency
Dark veins and old leaves with an extreme light green tint or shade

#6 Normal Leaf

#7 Early Signs of Nitrogen Deficiency.
Older leaves turn yellow, die and fall

#8 Potassium Deficiency
Very small holes with yellowish edges.

Thank you, that is really helpful!

Still though the leaves don't turn yellow at all. They go right to turning black.

In general, the plant is unhappy.
It has just come out of winter in a dark place.So it lacks energy (sunlight = energy). So it needs light. Then, as it was indoors, it probably was dry, using loads of water. The soil may have become quite alcaline, reducing nutrient availability & increasing salt content.

Were it my tree, I would do a gentle repot (No root trimming, just removing what is loose, give it a fresh batch of soil). Then put it in a sunny spot.

But you probably are better of in a gardening / plants forum. This really has nothing to do with growing bonsai.

Ok thank you for the advice! Indeed the leaves show signs that could indicate lack of nutrients so I will consider a repot to see if that helps.

I recently moved the plant to a window, and with lots of sunny weather the last days it has grown a lot of new leaves so that's a good sign.

Yes I do realise it's not a bonsai tree. I was not specifically looking for a bonsai so it's no big deal for me, but I will look for further advice another place and not bother you with my mallsai..