SSTP client disconnects shortly after successfully connected to VPN
Solution 1:
Eventually I found help for this issue here: http://support.microsoft.com/kb/947026 (Internet Archive backup).
I was lucky enough to fit into "Scenario 4".
Solution 2:
The SSTP-based VPN connection to the remote access server was terminated because of a security check failure sha1 and sha256 are two diferent algorithems with different keys.