Openvpn log Error: private key password verification failed

VyOS 1.3-rolling-202002011846

reset openvpn interface vtun0 adds this message to log:
Feb 27 17:50:05 vyos openvpn-vtun0[3070]: Error: private key password verification failed

But this is weird because ca.crt, vyos.crt were generated with nopass option

vtun0 tls section -
tls {
ca-cert-file /config/auth/ca.crt
cert-file /config/auth/vyos.crt
dh-file /config/auth/dh.pem
key-file /config/auth/vyos.key
}

Thank you for any hints to solving this puzzle.