I am having the very same problem. From my XP laptop I can connect easily. But from my MAC (Intel Mac OS X 10.4.8), no chance. The log shows:
Feb 12 18:25:56 InglezMac pppd[548]: pppd 2.4.2 (Apple version 233-13) started by root, uid 501
Feb 12 18:25:56 InglezMac pppd[548]: PPTP connecting...