Cannot raise Max number of concurrent openvpn clients

For same strange reason the number of concurrent clients that can connect to my openvpn server is limited to 70. No matter what parameter I change in server.conf the limit does not change. If I try to connect the 71st client errors like

openvpn needs a gateway parameter

or

dev tun also requires ifconfig

show up. If I restart openvpn server than the 71st client connects without errors.

There are a lot of UNDEF entries in my openvpn-status.log.

Here is my server.conf:

port 42000
proto udp
dev tun
ca /etc/openvpn/certs/ca.crt
cert /etc/openvpn/certs/server.crt
key /etc/openvpn/certs/server.key  # This file should be kept secret
dh /etc/openvpn/certs/dh1024.pem
server 10.9.0.0 255.255.0.0
ifconfig-pool-persist ipp.txt
keepalive 10 120
max-clients 300
persist-key
persist-tun
verb 3
client-config-dir /etc/openvpn/staticclients
client-to-client
route 10.9.0.0 255.255.0.0 10.9.0.1

and my typical client.conf:

client
dev tun
proto udp
remote host.domain.com 42000
resolv-retry infinite
nobind
persist-key
persist-tun
ca ca.crt
cert client-pc.crt
key client-pc.key
remote-cert-tls server
verb 3

Am I missing something?