I experienced the following issue:

LOGIN with Client 4.91 and 4.91 SP1 over IPX (clients installed IPX
only) on W2K and WinXP Workstations to a NW 6.5 Sp4 server gives an
unknown error (Lgnwnt32.dll: 870) and errorcode FFFF FA78 as long as
you are trying to login with correct credentials (you can repeat that
100 times).
But as soon as you have once entered a wrong credential the subsequent
login with the correct credential takes place absolutely problemless -
but this new login method: "first enter wrong password, hit enter, then
click away the error pop-up, afterwards login normally" is not really
estimated by the affected users.

With Client 4.90 Sp2 or before there are no login problems observed if
they login to a NW 6.5 SP4 server.

Same is true if 4.91 or later is used to connect to NW 6.5 Sp3 or
earlier - also no problems during login.

The culprit seems to be lgnwnt32.dll version dated 2005-01-14
or dated 2005-07-28 because backreving lgnwnt32.dll to version
dated 2004-12-14 of Client 4.90 psp2 "D" resolves the above explained

Maybe that is of help to somebody else with the same problem.

W. Prindl