@volker.weissmann, First of all, disable any systemd services related to login managers if you use such, reboot into the console mode, login, and run sudo NVIDIA-Linux-x86_64-455.38-custom.run. The installer should offer you to automatically disable nouveau and reboot, after that you will be able to install the driver without problems. If it doesn’t work for any reason, try to manually create /usr/lib/modprobe.d/disable-nouveau.conf file with the following content:
blacklist nouveau
options nouveau modeset=0
Reboot and restart the installer.