System: Vista 32 Bit Business, NW-Client (beta)

I can connect to an network with NW 6 servers.

I cannot connect to a network with NW 5.1 sp8 servers (eDirectory 8.7.3). I
can ping the servers but neither connect via client (tested with servername
and IP-Adress as servername) nor connect via "net use \\serverip\volume".

Did anyone connect to a nw 5.1 from vista successfully?