I made a small config change in the meantime because it looked like the primary DNS server being assigned through ppp is unresponsive.
Started using something like this:
[ipv4] method=auto dns=22.214.171.124; ignore-auto-dns=true
so that the bogus DNS nameservers returned by ppp-manager will not be used by NetworkManager. That, however, did not help. It definitely looks like openvpn cannot resolve vpn.resin.io.
Would there be much difference if I used a resinos.io image instead? Are they fulfilling similar roles now that -dev images are available through the dashboard?