Computer already was Windows 10 1803. Installed RU host. Worked once. Did remote administration, including a reboot. Couldn't connect again. Thought it was only the RU host not running. Went to the computer and found that it wasn't accessible at all. Had to hard reset it, but it didn't come up properly. Screen was blank, no action possible, nor reachable over network. Repeated tries had always the same end. Still worse: Since the carsh happened after the boot process, Windows saw it as improper shutdown and always did immediate normal boot.
Finally I found a solution: Repeatedly switch it off amidst booting. So it considered it boot fail and offered recovery options. I went into safe mode and disabled the RU service in service manager. (Can't uninstall things in safe mode.) Now the computer works again.