I have a new iMac with OS X 10.7 (Lion) on it that refuses to connect to a PPTP-based VPN server (running Windows 2003 SBS).

To shortcut past a lot of questions: There is a Dell workstation running Windows 7 on the same LAN as the Mac that is able to establish a PPTP connection to the same VPN server using the same credentials. That would seem to rule out any possible problems with the server, the port forwards on the server's firewall, the internet connection between the two, and the router local to the Dell and iMac.

Here's a "verbose" dump of the PPP log from the iMac:

Tue Sep  6 10:13:11 2011 : using link 0
Tue Sep  6 10:13:11 2011 : Using interface ppp0
Tue Sep  6 10:13:11 2011 : Connect: ppp0  socket[34:17]
Tue Sep  6 10:13:11 2011 : sent [LCP ConfReq id=0x1    ]
Tue Sep  6 10:13:11 2011 : PPTP port-mapping for en0, interfaceIndex: 0, Protocol: None, Private Port: 0, Public Address: 45f6f181, Public Port: 0, TTL: 0.
Tue Sep  6 10:13:11 2011 : PPTP port-mapping for en0 inconsistent. is Connected: 1, Previous interface: 4, Current interface 0
Tue Sep  6 10:13:11 2011 : PPTP port-mapping for en0 initialized. is Connected: 1, Previous publicAddress: (0), Current publicAddress 45f6f181
Tue Sep  6 10:13:11 2011 : PPTP port-mapping for en0 fully initialized. Flagging up
Tue Sep  6 10:13:14 2011 : sent [LCP ConfReq id=0x1    ]
Tue Sep  6 10:13:17 2011 : sent [LCP ConfReq id=0x1    ]
Tue Sep  6 10:13:20 2011 : sent [LCP ConfReq id=0x1    ]
Tue Sep  6 10:13:23 2011 : sent [LCP ConfReq id=0x1    ]
Tue Sep  6 10:13:26 2011 : sent [LCP ConfReq id=0x1    ]
Tue Sep  6 10:13:29 2011 : sent [LCP ConfReq id=0x1    ]
Tue Sep  6 10:13:32 2011 : sent [LCP ConfReq id=0x1    ]
Tue Sep  6 10:13:35 2011 : sent [LCP ConfReq id=0x1    ]
Tue Sep  6 10:13:38 2011 : sent [LCP ConfReq id=0x1    ]
Tue Sep  6 10:13:41 2011 : LCP: timeout sending Config-Requests
Tue Sep  6 10:13:41 2011 : Connection terminated.
Tue Sep  6 10:13:41 2011 : PPTP disconnecting...
Tue Sep  6 10:13:41 2011 : PPTP clearing port-mapping for en0
Tue Sep  6 10:13:41 2011 : PPTP disconnected

The error seems to be focused around the line, LCP: timeout sending Config-Requests, but I haven't had any luck in finding troubleshooting information for this. I've tried completely deleting the entire VPN "connection" from the Network prefpane and recreating it from scratch. I am certain the connection details are correct because they exactly match what successfully connects from the Win7 machine sitting next to the iMac.

Any suggestions?