Thank you for your quick response. I changed every wire and even tried it with another sensor - still the same problem. The voltage at the pi is 3.3V. I noticed that the problem occured after I changed the pull-up resistor of the 1-wire bus from 4k7 to 2k2 (before that everything worked as intended). Is there any intern pull-up resistor that I need to change or turn off/on?