I had this problem with my Apple TV 4K box right after my DSL modem was replaced with a new one. I figured out how to fix this while troubleshooting. In the modem settings, when I disabled the 5GHz wireless radio, devices must now connect via the 2.4GHz wireless radio. The Apple TV 4K now accepted the password and connected successfully to my WiFi network. When I disabled 2.4GHz, and enabled 5GHz the Apple TV 4K gets the "incorrect password" issue again. It seems when both 2.4GHz and 5GHz are enabled (the default setting), the Apple TV 4K was connecting via 5GHz (and getting the issue). This is really only a workaround until Apple fixes this issue with 5GHz connections and updates the software.