I'm also unable to access my Deluge WebUI - was using it yesterday evening then it just stopped opening after refreshing the page.
Logs show a warning whenever I try to open the web ui:
17:56:40 [WARNING ][deluge.ui.web.server :614 ] Failed to enable WebUI "normal" mode, script files are missing!
17:56:40 [WARNING ][deluge.ui.web.server :626 ] WebUI falling back to "debug" mode.
Currently running 2.0.5 - tried rolling back to 2.0.3 but that didn't help, and .old the appdata also didn't help.