Debian dropped --enable-iproute2 as suggested by upstream. Source:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=976070
For potential approach to research / fix this, see:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=976070#10
Maybe helpful search term:
openvpn unprivileged Netlink
The abstract thing to be found out is:
How to run OpenVPN unprivileged using netlink?
related user documentation wiki page: