Hello @binxbolling and welcome to the forum!
What happens? Nothing? Entry field cleared? Error message said somehow?
Alternatively, can you post any links describing exactly the symptom you see?
What I’m finding for some issues is workarounds and fixes, but it’s not clear your OMV plug-in is current. Last one released as OMV plug-in appears to be what Duplicati calls Release 2.0.3.3 (beta) 2018-04-02 and if this gets in OMV or plug-in specifics, forum help here may be limited. Did you reinstall using same version, done the same way using the plug-in system? Have you restarted Duplicati (or the entire NAS)?
Are you able and willing to install packages directly? I’m not sure how that would interact with the plug-in system, but Release: 2.0.4.5 (beta) 2018-11-28 is downloadable direct from Duplicati as a .deb package.
If you want to find your Duplicati version, but none of the above techniques let you in, you can see if your system has a /usr/lib/duplicati/changelog.txt on it. It’s also probably possible to remove the password by restarting Duplicati (maybe /usr/bin/duplicati or /usr/bin/duplicati-server – what does ps show up?) with:
–webservice-password: The password required to access the webserver. This option is saved so you do not need to set it on each run. Setting an empty value disables the password.
(quoting from the 2.0.4.5 help you can read if you run Duplicati with the help command as its command)
Uninstalling and reinstalling applies to Duplicati code, not Duplicati configuration (and you certainly would not want it otherwise, because you’d lose the backup databases commonly in ~/.config/Duplicati), so the password persistence is not a surprise. Do you have a configuration or backups you care about already?