It may work to install software that disable Windows enforcing of signed drivers.
I think that for me worked software EASYBCD. Go to its Advanced config and tick to Allow use of unsigned drivers. Reboot. I am not sure if reinstallation of the OpenPVN is necessary.
Today, mounting vhd/vhdx with built-in Windows tools (e.g. via Disk Management) still does not work for SMB-shared disk images, unless you have true SMB3 (e.g. NetApp works, but common basic Samba still does not support Resilient handles and thus fails — this is likely to be the root cause, not ext3.).
However, it is possible to mount such a remote vhd (but ...