I have exactly the same issue and try the same steps with the same result. But i manage to resolve it by removing the "openmediavault-omvextrasorg" and "openmediavault-openvpn" packages that i have installed and were not part of the official openmediavault repository.
Once a removed this packages the problem got away for me.
For removing the "openmediavault-omvextrasorg" package a had to modify the file:
and comment the "omv-mkconf" command in line 35 as this command is not anymore in OMV5.
Hope that this helps