ISA Server 2006: no network protocols were successfully negotiated

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http://serverfault.com – I've upgraded one old and reliable ISA Server 2004 to 2006 (running on WS2003R2, nothing else runs on that server), it is an edge firewall and VPN provider. Later this year, it will be replaced by Forefront (and new hardware), but now, I need to keep it running. This upgrade was due to devices that require L2TP. After the upgrade, whatever I do, VPN doesn't work - L2TP, PPTP - from logs I can see user correctly authenticates, gets IP from DHCP server, and then the connection fails, on the following error in Event log (on ISA Server): The user [...] connected to port VPN3-4 has been disconnec (HowTos)