You execute:

systemctl start xrdp

and the service does not start and you find the following in the messages log

Apr 7 16:01:22 myserver systemd: Started xrdp session manager.
Apr 7 16:01:22 myserver systemd: Starting xrdp session manager...
Apr 7 16:01:22 myserver systemd: Started xrdp daemon.
Apr 7 16:01:22 myserver systemd: Starting xrdp daemon...
Apr 7 16:01:22 myserver systemd: xrdp.service: main process exited, code=exited, status=1/FAILURE
Apr 7 16:01:22 myserver systemd: Unit xrdp-sesman.service is not needed anymore. Stopping.
Apr 7 16:01:22 myserver systemd: Unit xrdp.service entered failed state.
Apr 7 16:01:22 myserver systemd: xrdp.service failed.
Apr 7 16:01:22 myserver systemd: Stopping xrdp session manager...
Apr 7 16:01:22 myserver systemd: Stopped xrdp session manager.

Execute the following commands

xrdp-keygen xrdp auto 2048
systemctl start xrdp

and you should be up again