I’m not finding reports of that exact message in this forum or GitHub issues. You mentioned other reports?
Yes, but maybe not the most recent version available directly. Maybe you need to ask for a newer one like
Duplicati 2.0.3.3 was requested three days after its release. That post also says one can directly install, if there is not a plug-in, meaning (if it’s correct) you can install 2.0.4.5 directly if you don’t have that, and you can probably find out if you have that from the changelog.txt I described, assuming you have shell access.
The OpenMediaVault Plugin developers sound like they don’t want to keep up with Duplicati releases, but possibly a case could be made if there’s a generic password problem with 2.0.3.3 (and there is, however failure presents differently than your message from what I can tell). Still need to debug what’s going on…
Your other instance of Duplicati server probably needs to be stopped first, which requires using systemctl from your OpenMediaVault 4 installation which is described as very close to Debian 9, known as stretch. systemd - system and service manager is more information on how to deal with services on your system.
Because you want to remove Duplicati thoroughly, you can also look for the config directory I mentioned to see if you can remove it underneath the running Duplicati. If so, then a reboot shouldn’t have a password. Going on without one can find the Duplicati version in About on the home screen. This password can be added and removed at any time in Settings on the home screen (but possibly you’ll get a lock-out again).