Just installing a CentOS 5.5 x64 PPTPD server.
Remotely connected using a pptp client and could connect and login but not do an ls on a directory of files.
Reduce the MTU
edit /etc/sysconfig/network-scripts/ifcfg-eth0 (or whichever is your ethernet adaptor running pptpd) and add MTU= the value should be whatever the maximum size you can get away with.
# Realtek Semiconductor Co., Ltd. RTL8111/8168B PCI Express Gigabit Ethernet controller DEVICE=eth0 BOOTPROTO=none HWADDR=20:cf:30:XX:XX:XX ONBOOT=yes DHCP_HOSTNAME=myhostname.mydomain.local IPADDR=192.168.59.60 NETMASK=255.255.255.0 GATEWAY=192.168.59.254 TYPE=Ethernet # added by james because ls ps vi top where hanging # as soon as I tried to return more than a page of text # manual command is: # ip link set dev eth0 mtu 1200 MTU=1200