I tested it today and charged my Xtorm AL480N from 75% to 100% with Xtorm AP275 SolarBooster (2.10A, 21W) without any problems. The weather was sunny but cloudy, partly cloudy and the sun was in mid-April. The charging current was between 1.8 and 2.4 A, so Knubebus' statement that it needs 3 A is clearly wrong.

So I confirm that this is compatible with Xtorm AP275 SolarBooster (2.10A, 21W).

I think the Xstorm Solar Booster (has 2.1A) is exactly matched to this powerbank. Doesn't the manufacturer write this in the description if it's technically impossible, right? What's right now?

Unfortunately no - the output voltage of 2.1A is not enough - minimum input is 3A - I bought both products together and it does not work. It charges for 2sec and stops again.