I have a DrayTek 2820 and needed to allow a colleague to access my network. He created a VPN client (PPTP) on Windows XP SP3 and I configured the DrayTek to allow him access (same username and password). Whatever we did, he was unable to connect and he received an 800 error. I did not see any evidence of his incoming connection in "VPN and Remote Access" > "Connection Management". We both had PPTP selected, along with PAP or CHAP (I know this is insecure, but we just wanted to get it set up before beefing up the security).

Is it possible to use the XP client to connect to the DrayTek or would he need a specific client installed on his PC?

EDIT:

I've managed to resolve this and thought it would be sensible to post the solution, just in case anyone else ran into the same problem.

tried a factory reset, having made a backup copy of the current configuration. I re-entered the ADSL logon credentials but it wouldn't connect to the ISP. I called them and they confirmed my username and password were correct and they could see that my router was trying to connect but it was failing.

I rolled back the settings using the backup file that I had generated but it still wouldn't connect to the internet, though it had done so earlier today! At this time, I thought my DrayTek was bricked so I upgraded the firmware. The last version was dated mid-2010 (I can't recall the exact version) but it's now v3.3.5.2_232201. Fortunately, I can connect to the internet now and, more importantly, I can connect to my DrayTek via PPTP and L2TP VPNs. I can't see anything in the release notes about this firmware doing anything significant to the VPN functionality. I wonder if the odd behaviour that I saw was due to the firmware having become corrupt somewhere so, by overwriting it with a more recent version, it's corrected the problem.