You didn’t mention what version of Duplicati you were running, but is it possible you’re running more than one instance (thus causing the locked database)?
Personally, I went from 2.0.2.1 beta client install to a service install then through 3 canary versions and on 2.0.2.9 things totally blew up and I had to uninstall Duplicati all together before a re-install would even let the GUI or service start up.
Through all those updates I think I found (but ultimately destroyed) an scenario of both the service running as well as the client - each with their own server instance. So even though I had it all working with the service at one point, an update (or something I did) split that out into using two different server runs.