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?