Got an odd one for you.

Using a Draytek 2760Vn as the primary router for a fixed wireless service at my parents home (note: router, the dish acts as some sort of magical modem).
Living in a different part of the country means I have to VPN into the router to administer the various machines through VNC; this was all going well until VoIP problems forced us to switch to the locked down router provided by the ISP (no VPN functionality).

On swapping the Draytek back in a month or so later (with no change in config on router or at ISP), I discovered that after VPN connections from a remote dial in user simply time out. Checking the Syslog shows no activity at all for VPN, not a single instance indicating misconfiguration or failed handshake etc.

Factory reset, upgraded the firmware, reconfigured both ends of the equation, got a friend to try dial in: still no connection, timing out after about a minute.

Interestingly, said friend has exactly the same problem with VPN'ing into his office's Draytek since a few days ago.

Does anyone have any recommendations on what steps we should try next?