Connect to local network with OpenVPN . Problem: Local IP address unset but adding route?
I managed to resolve what I think was the same issue as you (I am also using Shibby firmware for OpenVPN and using an Android client).

I believe it comes down to how the routes are pushed to the client and the Android app not liking it (the refusal to open tun interface is an error message belonging to the app).

I'm not sure why the routes aren't pushed successfully, but if you set up your client profile under IP and DNS, disable Pull Settings and set the IPv4 address to be the same network/cidr as whatever bridge your TAP OpenVPN interface is (default is br0 and 192.168.1.0/24).

This also even gives me internet access over the router.

Note I'm using the latest versions of the Android app and Shibby firmware as of writing.