OpenVPN TAP interface: gateway options

I'm setting up an OpenVPN TAP tunnel between a pfSense (2.4.0) server and a LEDE (17.01.4) client.

If I import the pfSense-generated .ovpn file in LEDE, I get the following error:

Wed Nov 8 01:22:54 2017 TUN/TAP device tap0 opened
Wed Nov 8 01:22:54 2017 NOTE: unable to redirect default gateway -- VPN gateway parameter (--route-gateway or --ifconfig) is missing
Wed Nov 8 01:22:54 2017 Initialization Sequence Completed

So, I add "route-gateway " to the .ovpn file. However, I then get the following error:

Wed Nov 8 01:24:05 2017 TUN/TAP device tap0 opened
route: SIOCADDRT: File exists
Wed Nov 8 01:24:05 2017 ERROR: Linux route add command failed: external program exited with error status: 1
route: SIOCADDRT: Network unreachable
Wed Nov 8 01:24:05 2017 ERROR: Linux route add command failed: external program exited with error status: 1
route: SIOCADDRT: Network unreachable
Wed Nov 8 01:24:05 2017 ERROR: Linux route add command failed: external program exited with error status: 1
route: SIOCADDRT: Network unreachable
Wed Nov 8 01:24:05 2017 ERROR: Linux route add command failed: external program exited with error status: 1
Wed Nov 8 01:24:05 2017 Initialization Sequence Completed

So somehow, LEDE wants the 'route-gateway' option, but then doesn't recognize it when I define it? For reference: using the .ovpn file on a Windows client with the route-gateway option results in a perfectly working connection.

What am I missing?