I posted this over in the NMAS forum but it is probably relevant here.

I recently applied NW6.5 SP5 to our main file/print server. (was at
SP3). There were already other servers in the tree at this support
pack. After installing the support pack we had a large number of users
that were unable to login.They would get the error message "The system
could not log you into the network. Make sure your user name and
connection information are correct".

This was happening with client V4.91SP2 and 4.90SP2. The common thing
with all these machines were that the NW client had a server name in
the server field.(same server that had just been upgraded) Removing
this allowed them to log on.

I did an NMAS trace on the server and got the following.

749: NCPCheckIfLocalUser: client supplied user DN CN=xx.OU=xx.O=xx
749: NCPCheckIfLocalUser: checking actual user DN CN=xx.OU=xx.O=xx
749: NCPCheckIfLocalUser is a local user.
749: Server thread started
749: NCP client address 0 0 0 1
749: Set net address of size 20
749: sendProxyClientInfo: not remote session
749: Free caller supplied net address
749: >>ServerGet: message size=8 queue size 0
729: ERROR: -1644 NMAS Manager
729: <<ServerPut: message size=8 queue size 0

It appears that the NCP client address is incorrect. I assume this
address is handed out by the client.

Any thoughts on why this is occurring?