Welcome to Permies, Martin!
Coriander (also known as cilantro) is a short-lived plant and you are doing nothing wrong. It sends up those "anemic stalks" when it is getting ready to flower, set seed, and die. You just have to let it go to seed, pick the seed and start a new pot on the window shelf, and the cycle repeats itself. In a cool, northern climate like Berlin, you probably get one growth cycle per year outside in the garden. Since coriander is a cold-hardy plant, where I am, it grows slowly all through the winter, and then when spring comes, I can get 3, maybe 4 cycles during the hot months.
If you keep it outside (do you have a south facing balcony?) you may be able to keep it bushy all through the winter. But once it has bolted and sent up some anemic stalks, you can't turn the clock back and reset it to the bushy mode. It
should be hardy down to about -10C, however, if you keep it protected or covered, the temperature can get even colder and it will still survive. The protection can be as simple as a PET plastic bottle with the bottom cut off sitting on top of the pot.
