Debian – openvpn – client can only connect after service restart

I have 3 Windows clients which connect to an openVPN server on a Debian 6.0 server. The connection can be established for each one of them without errors, however, if I want to disconnect client1 and connect client2 the openvpn service on the server must be restarted..and this is only possible if I comment ifconfig-pool-presist ipp.txt, which is quite annyoing, restarting every time another client wants to connect. The first connected client after a restart however can connect and disconnect as much as desired.

If I do not restart the openvpn service before connecting with a different client, openVPN Gui 2.3 shows the connection as established, but ping is not possible.

Does anybody have an idea what might cause this strange behaviour / where I could start looking?

Below is my server conf (as you can see I increased the max-clients number, just in case this could be the problem, but it didn't fix it..)

port 1194
proto udp
dev tun1
topology subnet
ifconfig-noexec
route-noexec
ca /etc/openvpn/ca.crt
cert /etc/openvpn/server.crt
key /etc/openvpn/server.key
dh /etc/openvpn/dh1024.pem
server 10.240.43.0 255.255.255.0
;ifconfig-pool-persist ipp.txt
push "dhcp-option DNS 10.240.43.1"
keepalive 10 120
comp-lzo
max-clients 100
persist-key persist-tun
status openvpn-status.log
log        /etc/openvpn/openvpn.log
verb 6

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Edit: additional client configuration, see openvpn: connection established, can't ping server tun interface (debian server, windows & os x clients)

client
dev tun1
(rename openvpn network on Windows machine if there are more than one present)
dev-node openVPN
proto udp
remote [public-ip-address-of-my-server] 1194
(Dummy default gateway to work around Windows 'unidentified
network'/'unknown network'):
route-metric 50 route 0.0.0.0 0.0.0.0 10.240.43.1
resolv-retry infinite
nobind
persist-key
persist-tun
ca "C:\Program Files (x86)\OpenVPN\config\ca.crt"
cert "C:\Program Files (x86)\OpenVPN\config\client1.crt"
key "C:\Program Files (x86)\OpenVPN\config\client1.key"
ns-cert-type server
comp-lzo verb 3

openVPN GUI is started with Admin rights to allow adding of routes

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Any help is greatly appreciated! Thank you!