Hi Matthias
Did you check if the protocol used is PPTP ?, windows has another VPN protocol that only works with microsoft, I think that it's called L2TP (not sure).
disable PPTP compression, or won't work also.
did you, in server-manager / remote access, set at least to 1 (one) the option
"Number of PPTP clients" ?
this one should let you login at least.
if you can login but still not seeing the remote network, it could be a
local/remote IP's problem, I had this problem between my SME@home
and the server @ work, since both ÍP segments where identical
192.168.0.x, you need different IP portions in the lan than in the VPN
that you are trying to connect, since this is necessary so your
machine will know the difference, if both ip segments where the same,
windows will try to route everything locally instead of using the vpn.
I'm not sure if i explain it clearly, not very good for that

see you
byby
Light