here is a picture of the plant, a shefflera.
It has some very sticky transparent liquid on many leaves, and also brown spots. The spots themselves are kind of "dry" and mostly located at the leaves knots.
It seems that the entire plant is becoming infested by this problem.
Searching for answers, I've found this question about "sticky citrus leaves", that looks very similar. But it's not a citrus, so I made my own question.
citrus question link: How to cure sticky citrus leaves?