Have a Macintosh (10.8.2) that connects just fine to a CentOS 6.0 virtual private server (OpenVZ, with PPP added by the host) via PPTP.

A Windows 7 Home Premium client (virtualized in Sun's Virtual Box), on the same computer, using the same Ethernet connection, cannot connect to the Linux VPN server.

I have iptables disabled (for testing) on the Linux box. I have the Windows firewall turned off. /var/log/messages looks like this, for a Windows connection:

Oct 12 18:44:30 production pptpd[1880]: CTRL: Client 66.104.246.168 control connection started
Oct 12 18:44:30 production pptpd[1880]: CTRL: Starting call (launching pppd, opening GRE)
Oct 12 18:44:30 production pppd[1881]: Plugin /usr/lib/pptpd/pptpd-logwtmp.so loaded.
Oct 12 18:44:30 production pppd[1881]: pptpd-logwtmp: $Version$
Oct 12 18:44:30 production pppd[1881]: pppd options in effect:
Oct 12 18:44:30 production pppd[1881]: debug#011#011# (from /etc/ppp/options.pptpd)
Oct 12 18:44:30 production pppd[1881]: nologfd#011#011# (from /etc/ppp/options.pptpd)
Oct 12 18:44:30 production pppd[1881]: dump#011#011# (from /etc/ppp/options.pptpd)
Oct 12 18:44:30 production pppd[1881]: plugin /usr/lib/pptpd/pptpd-logwtmp.so#011#011# (from command line)
Oct 12 18:44:30 production pppd[1881]: require-mschap-v2#011#011# (from /etc/ppp/options.pptpd)
Oct 12 18:44:30 production pppd[1881]: refuse-pap#011#011# (from /etc/ppp/options.pptpd)
Oct 12 18:44:30 production pppd[1881]: refuse-chap#011#011# (from /etc/ppp/options.pptpd)
Oct 12 18:44:30 production pppd[1881]: refuse-mschap#011#011# (from /etc/ppp/options.pptpd)
Oct 12 18:44:30 production pppd[1881]: name pptpd#011#011# (from /etc/ppp/options.pptpd)
Oct 12 18:44:30 production pppd[1881]: pptpd-original-ip 66.104.246.168#011#011# (from command line)
Oct 12 18:44:30 production pppd[1881]: 115200#011#011# (from command line)
Oct 12 18:44:30 production pppd[1881]: lock#011#011# (from /etc/ppp/options.pptpd)
Oct 12 18:44:30 production pppd[1881]: local#011#011# (from command line)
Oct 12 18:44:30 production pppd[1881]: novj#011#011# (from /etc/ppp/options.pptpd)
Oct 12 18:44:30 production pppd[1881]: novjccomp#011#011# (from /etc/ppp/options.pptpd)
Oct 12 18:44:30 production pppd[1881]: ipparam 66.104.246.168#011#011# (from command line)
Oct 12 18:44:30 production pppd[1881]: proxyarp#011#011# (from /etc/ppp/options.pptpd)
Oct 12 18:44:30 production pppd[1881]: 192.168.97.1:192.168.97.10#011#011# (from command line)
Oct 12 18:44:30 production pppd[1881]: nobsdcomp#011#011# (from /etc/ppp/options.pptpd)
Oct 12 18:44:30 production pppd[1881]: require-mppe-128#011#011# (from /etc/ppp/options.pptpd)
Oct 12 18:44:30 production pppd[1881]: mppe-stateful#011#011# (from /etc/ppp/options.pptpd)
Oct 12 18:44:30 production pppd[1881]: pppd 2.4.5 started by root, uid 0
Oct 12 18:44:30 production pppd[1881]: Using interface ppp0
Oct 12 18:44:30 production pppd[1881]: Connect: ppp0 <--> /dev/pts/1

(At this point the Windows machine displays a dialog, reading: "Verifying user name and password...")

Oct 12 18:45:00 production pppd[1881]: LCP: timeout sending Config-Requests
Oct 12 18:45:00 production pppd[1881]: Connection terminated.
Oct 12 18:45:00 production pppd[1881]: Modem hangup
Oct 12 18:45:00 production pppd[1881]: Exit.
Oct 12 18:45:00 production pptpd[1880]: GRE: read(fd=6,buffer=8059660,len=8196) from PTY failed: status = -1 error = Input/output error, usually caused by unexpected termination of pppd, check option syntax and pppd logs
Oct 12 18:45:00 production pptpd[1880]: CTRL: PTY read or GRE write failed (pty,gre)=(6,7)
Oct 12 18:45:00 production pptpd[1880]: CTRL: Client 66.104.246.168 control connection finished

The Macintosh connecting looks like this in /var/log/messages:

Oct 12 18:50:49 production pptpd[1920]: CTRL: Client 66.104.246.168 control connection started
Oct 12 18:50:49 production pptpd[1920]: CTRL: Starting call (launching pppd, opening GRE)
Oct 12 18:50:49 production pppd[1921]: Plugin /usr/lib/pptpd/pptpd-logwtmp.so loaded.
Oct 12 18:50:49 production pppd[1921]: pptpd-logwtmp: $Version$
Oct 12 18:50:49 production pppd[1921]: pppd options in effect:
Oct 12 18:50:49 production pppd[1921]: debug#011#011# (from /etc/ppp/options.pptpd)
Oct 12 18:50:49 production pppd[1921]: nologfd#011#011# (from /etc/ppp/options.pptpd)
Oct 12 18:50:49 production pppd[1921]: dump#011#011# (from /etc/ppp/options.pptpd)
Oct 12 18:50:49 production pppd[1921]: plugin /usr/lib/pptpd/pptpd-logwtmp.so#011#011# (from command line)
Oct 12 18:50:49 production pppd[1921]: require-mschap-v2#011#011# (from /etc/ppp/options.pptpd)
Oct 12 18:50:49 production pppd[1921]: refuse-pap#011#011# (from /etc/ppp/options.pptpd)
Oct 12 18:50:49 production pppd[1921]: refuse-chap#011#011# (from /etc/ppp/options.pptpd)
Oct 12 18:50:49 production pppd[1921]: refuse-mschap#011#011# (from /etc/ppp/options.pptpd)
Oct 12 18:50:49 production pppd[1921]: name pptpd#011#011# (from /etc/ppp/options.pptpd)
Oct 12 18:50:49 production pppd[1921]: pptpd-original-ip 66.104.246.168#011#011# (from command line)
Oct 12 18:50:49 production pppd[1921]: 115200#011#011# (from command line)
Oct 12 18:50:49 production pppd[1921]: lock#011#011# (from /etc/ppp/options.pptpd)
Oct 12 18:50:49 production pppd[1921]: local#011#011# (from command line)
Oct 12 18:50:49 production pppd[1921]: novj#011#011# (from /etc/ppp/options.pptpd)
Oct 12 18:50:49 production pppd[1921]: novjccomp#011#011# (from /etc/ppp/options.pptpd)
Oct 12 18:50:49 production pppd[1921]: ipparam 66.104.246.168#011#011# (from command line)
Oct 12 18:50:49 production pppd[1921]: proxyarp#011#011# (from /etc/ppp/options.pptpd)
Oct 12 18:50:49 production pppd[1921]: 192.168.97.1:192.168.97.10#011#011# (from command line)
Oct 12 18:50:49 production pppd[1921]: nobsdcomp#011#011# (from /etc/ppp/options.pptpd)
Oct 12 18:50:49 production pppd[1921]: require-mppe-128#011#011# (from /etc/ppp/options.pptpd)
Oct 12 18:50:49 production pppd[1921]: mppe-stateful#011#011# (from /etc/ppp/options.pptpd)
Oct 12 18:50:49 production pppd[1921]: pppd 2.4.5 started by root, uid 0
Oct 12 18:50:49 production pppd[1921]: Using interface ppp0
Oct 12 18:50:49 production pppd[1921]: Connect: ppp0 <--> /dev/pts/1
Oct 12 18:50:52 production pppd[1921]: MPPE 128-bit stateless compression enabled
Oct 12 18:50:52 production pppd[1921]: Unsupported protocol 'IPv6 Control Protocol' (0x8057) received
Oct 12 18:50:52 production pppd[1921]: Unsupported protocol 'Apple Client Server Protocol Control' (0x8235) received
Oct 12 18:50:52 production pppd[1921]: Cannot determine ethernet address for proxy ARP
Oct 12 18:50:52 production pppd[1921]: local  IP address 192.168.97.1
Oct 12 18:50:52 production pppd[1921]: remote IP address 192.168.97.10
Oct 12 18:50:52 production pppd[1921]: pptpd-logwtmp.so ip-up ppp0 chris 66.104.246.168

I'm baffled...