Yeah Daisy is right, not enough roots for the size of the cactus. How does the root mass compare to what it looked like when you repotted it? These kind of cacti normally make quite a large mass of roots. What is there looks relatively good, so it should be able to recover. You said you watered it last 3 weeks ago how was the soil when you took the plant out of it. Completely dry or still moist?
The green part of the cactus looks surprisingly good for a plant with so little roots. If the soil was quite dry after 3 weeks I think you might be able to water it somewhat more frequently as long as the cactus is fairly warm and you provide it plenty of sun light.
@Vlasta for this kind of thing it is probably better to start a new thread. I think it is likely some kind of fungus due to the plant being wet and relatively cold with very little to no airflow. You might be able to save the unaffected plants - unpot it all remove the affected plants and clean the remaining stems with soapy water try to get as much of the soil off and then repot in fast draining soil then provide as much light as possible and do not water the plant unless the soil is completely dry, if at all possible try to provide some air circulation around the plant.
