Some updates:
I tried out different authentication protocols (MS-CHAP v1/v2, CHAP) both on
the server side as well as on the client but it still doesn't work. If I
monitor the traffic on the isa server I can see the initiation of the pptp
connection and exactly 30 seconds later the connection will be disconnected.
There seems to be no authentication yet cause we use RADIUS and no RADIUS
packets can be seen on the firewall.
With Windows XP clients everything works fine with exact the same VPN
settings on this ISA server.
Also very important: With the same Vista machine I can connect to a Draytek
VPN-router over PPTP without problems.
Therefore there must be a problem between the PPTP-implementation of Vista
and the ISA server.
I tried to have a look on the client logs, but I cannot find this function
any longer?! The diagnostic only tells me that no problem can be detected.
What a stupid answer if the same system tells me that the vpn connection
could not be established. Yesterday at home (with WLAN instead of Ethernet) I
saw a diagnostic report function and it told me something like error 807.
Perhaps I am only too stupid to find that report again ...
Peer