AC68 -Merlin 378.56 beta2- OpenVPN can't start

[winter]

Hi, just registered and have a question on starting OpenVPN server on my AC68 with 378.56 beta 2.
If I run OpenVPN with Authorization Mode set to TLS, it runs fine.

I also want to run a OpenVPN with Authorization Mode set to Static Key, it appears the server can not be started. below is the router's general log. I am not sure if I am running some wrong setting or what.

Please take a look. Thanks.

Router log:
Oct 22 21:34:00 openvpn[1406]: Socket Buffers: R=[87380->87380] S=[16384->16384]
Oct 22 21:34:00 openvpn[1406]: TUN/TAP device tun22 opened
Oct 22 21:34:00 openvpn[1406]: TUN/TAP TX queue length set to 100
Oct 22 21:34:00 openvpn[1406]: do_ifconfig, tt->ipv6=0, tt->did_ifconfig_ipv6_setup=0
Oct 22 21:34:00 openvpn[1406]: /sbin/ifconfig tun22 10.16.0.1 netmask 10.16.0.2 mtu 1500 broadcast 255.255.255.253
Oct 22 21:34:00 kernel: ADDRCONF(NETDEV_CHANGE): tun22: link becomes ready
Oct 22 21:34:00 openvpn[1406]: Linux ifconfig failed: external program exited with error status: 1
Oct 22 21:34:00 openvpn[1406]: Exiting due to fatal error
Oct 22 21:34:21 openvpn[698]: event_wait : Interrupted system call (code=4)
Oct 22 21:34:21 openvpn[698]: TITLE,OpenVPN 2.3.8 arm-unknown-linux-gnu [SSL (OpenSSL)] [LZO] [EPOLL] [MH] [IPv6] built on Oct 17 2015
Oct 22 21:34:21 openvpn[698]: TIME,Thu Oct 22 21:34:21 2015,1445567661
Oct 22 21:34:21 openvpn[698]: HEADER,CLIENT_LIST,Common Name,Real Address,Virtual Address,Bytes Received,Bytes Sent,Connected Since,Connected Since (time_t),Username
Oct 22 21:34:21 openvpn[698]: HEADER,ROUTING_TABLE,Virtual Address,Common Name,Real Address,Last Ref,Last Ref (time_t)
Oct 22 21:34:21 openvpn[698]: GLOBAL_STATS,Max bcast/mcast queue length,0
Oct 22 21:34:21 openvpn[698]: END
Oct 22 21:35:13 openvpn[698]: event_wait : Interrupted system call (code=4)
Oct 22 21:35:13 openvpn[698]: TITLE,OpenVPN 2.3.8 arm-unknown-linux-gnu [SSL (OpenSSL)] [LZO] [EPOLL] [MH] [IPv6] built on Oct 17 2015
Oct 22 21:35:13 openvpn[698]: TIME,Thu Oct 22 21:35:13 2015,1445567713
Oct 22 21:35:13 openvpn[698]: HEADER,CLIENT_LIST,Common Name,Real Address,Virtual Address,Bytes Received,Bytes Sent,Connected Since,Connected Since (time_t),Username
Oct 22 21:35:13 openvpn[698]: HEADER,ROUTING_TABLE,Virtual Address,Common Name,Real Address,Last Ref,Last Ref (time_t)
Oct 22 21:35:13 openvpn[698]: GLOBAL_STATS,Max bcast/mcast queue length,0
Oct 22 21:35:13 openvpn[698]: END