Following the instructions on setting up a webcam, I ran
journalctl -u duetpluginservice -f
Here's the results of that:
-- Logs begin at Thu 2019-02-14 04:11:59 CST. --
May 30 09:17:22 duet3 systemd[1]: Starting Duet Plugin Service...
May 30 09:17:24 duet3 DuetPluginService[921]: Duet Plugin Service v3.4.0
May 30 09:17:24 duet3 DuetPluginService[921]: Written by Christian Hammacher for Duet3D
May 30 09:17:24 duet3 DuetPluginService[921]: Licensed under the terms of the GNU Public License Version 3
May 30 09:17:27 duet3 DuetPluginService[921]: [info] DuetPluginService.Program: Settings loaded
May 30 09:17:28 duet3 DuetPluginService[921]: [info] DuetPluginService.Program: Connection established
May 30 09:17:30 duet3 DuetPluginService[921]: [info] DuetPluginService.Program: Plugin MotionWebcamServer loaded
May 30 09:17:30 duet3 systemd[1]: Started Duet Plugin Service.
I'm not an expert, but I don't see anything unusual there.