I'm trying to make a VPN connection from my Windows XP at branch office to my Windows 2003 server at my head office. Once in a blue moon I can actually connect. But most times it says "verifying username and password" for 30 or 40 seconds and then ends with an error 721. I don't understand why it doesn't connect most of the time but does connect occasionally.
We are using xp-sp2 and vista. we are implemented PPTP vpn.
I've read that this is caused by either your ISP, your router, usage of NAT on your router, your OS, or firewall ports on the corporate office blocking some ports that are using the PPTP VPN process. With all these variables, we definitely need a bit more information.
Are you the only one experiencing this?
How do you connect to the Internet?
Some of these can be easily isolated, whereas others can't.
RRAS does log detailed information in C:\Windows\tracing, and standard logging on C:\Windows\System32\Logfiles on the RRAS server itself or the RADIUS server (depending on your setup). Maybe the logs there can help.
KiLLaBeE is absolutely right, this issue can be caused by many variables but one in particular I'd like to add on is the ISP. Your ISP only guarantees you browsing capabilities. Your ISP does not have to allow FTP, VPN, Email ect... Unless you have a business connection, the fine print states only browsing is promised.
So, if your connection is intermittent your more than likely looking at an ISP issue. But I would still check your router, NAT settings ect as KiLLaBeE suggested.
Windows PPTP VPN makes use of TCP port 1723 and GRE tunnelling. You must ensure both routers have PPTP pass-through options enabled. If you've got Cisco routers on each end, then things are much more easy as you can terminate your PPTP VPN directly on them.
I'd also check any possible 'Firewall' features that might be enabled on your routers, as they tend to become a common problems in these situations.