I had basically the same issues on latest ayufan stable image (0.7.9. release) -rock64.
In short, tried configuring through interfaces.d , deleting that and configuring directly in /etc/interfaces, no luck. Web GUI even with fresh installation , no luck. OMV first aid didn't work for that with same error as OP.
In the "end", I've left a messy configuration due to lack of time and will at the time, DHCP assigned address plus a static one configured through CLI (interfaces.d) (needed for Pi-Hole).
As I've bumped into this thread, it "forced" me to play with it again. I downloaded "regular" OMV image from sourceforge, and everything was fine.
Configured Static IP through WebGUI with no errors. Even managed to install Plex plugin through GUI, didn't work before, also had to use CLI. Icing on the cake, solved my VPN problems as well. (That one I still installed through terminal because I found it more straightforward.)
Given how easy it is to setup OMV through WebGUI, it didn't take all that much time I thought it would.