This error usually only occurs on routers. The code lines over and under the error message should look something like:
unknown daemon.notice openvpn: /sbin/ifconfig tun11 add 2a03:8600:1003:301:313b:3842:f0e3:8fb1/64
unknown daemon.err openvpn: Linux ifconfig inet6 failed: external program exited with error status: 1
unknown daemon.notice openvpn: Exiting due to fatal error
The error occurs if the device doesn’t support IPv6.
In order to solve the problem, you need to deactivate IPv6 on your OVPN dashboard. After doing this, try reconnecting and it should work.