Thanks for all your suggestions! I isolated the plants for now (and moved them away from the plant light) and I will try to get rid of the affected leaves gradually.
I noticed small white dots on the leaves, which makes me think that it might actually be lice. Although I never saw one move, so not sure about that.
Hopefully my poor jade plants will recover.
Wenn man nur die Beilagen von den Fleischgerichten isst fehlen halt oft Proteine. Ich würde da glaube ich nicht satt werden. Aber für ab und zu ist es sicher praktisch.