Yes, install plugin manually as this is an issue often reported(don't know if proper bugticket ever was submitted to the bugtracker about it, but still unfixed regardless) and happens with some plugins when using the webui install feature. Just place plugin into dockers /config mounted profile dir ...

Running in docker with linuxserver/deluge Deluge version: 2.0.3-2-201906121747-ubuntu18.04.1 Python: 3.6.9 When I install the plugin via the web UI I see a new "file" in the plugins directory (/config/plugins) called "[object FileList]" but the plugin is not listed in the plugins...