i have the same problem my webgui disappeared after installing subsonic. So i removed it but via cli but that didnt fix the problem.
The filesize of my config.xml is 0
-rw------- 1 openmediavault openmediavault 0 Apr 14 16:45 /etc/openmediavault/config.xml
Server is running fine just no webgui
Any help how to restore ? ( no .bak file in folder)