openvpn: works great on raspberry pi 3B, not always doesn't work on raspberry pi 3B plus

installed version 4.1.7 on both 3B and 3B+. openvpn works great on 3B, but always not working on 3B+: client can't connect with server. Other functions seem OK on both.

ifconfig -a shows below difference, didn't see vpn network on 3B+, how to fix it?

on raspberry pi 3B

tun0: flags=4305<UP,POINTOPOINT,RUNNING,NOARP,MULTICAST> mtu 1500
inet 10.8.0.1 netmask 255.255.255.0 destination 10.8.0.1
unspec 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00 txqueuelen 100 (UNSPEC)
RX packets 882 bytes 128654 (125.6 KiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 1305 bytes 1031929 (1007.7 KiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

on raspberry pi 3B+

lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
inet 127.0.0.1 netmask 255.0.0.0
inet6 ::1 prefixlen 128 scopeid 0x10<host>
loop txqueuelen 1000 (Local Loopback)
RX packets 138 bytes 15210 (14.8 KiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 138 bytes 15210 (14.8 KiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0