I am prompted for credentials when accessing a particular server after I am already logged into the domain. Other servers grant me access without prompting. No credentials I enter are accepted. Why?

 It is possible you are connecting to a NAS (Network Attached Storage) device. Some NAS devices seem to only support using Kerberos in the extended security protocol and not NTLM extended security.

This means that unless the user has already gotten Kerberos credentials, either by
logging in with ADmitMac at the OSX login window, or using Apple's
Kerberos with ADmitMac/DAVE 5 for OSX, then the user will have to use the DAVE( OS9 or 10 ) or ADmitMac's 'Policy' configuration tool to drop the security level down to the lowest level( Send LM & NTLM responses ).

To rephrase, with DAVE5 or ADmitMac, you must have Kerberos credentials if you want to connect to Filer at a security level higher than the lowest setting.

If you are using ADmitMac, go to /Applications/Utilities and open the Directory Access utility. Under the "Services" tab, double click on ADmitMac. In the ADmitMac Directory Access plugin window that appears, click on the "Policies" tab. Change the LanMan Policy to the following option:

"Send LM & NTLM responses
Client will only use LM & NTLM authentication and never use NTLMv2 or Kerberos."

If you are using DAVE, go into the System Preferences and click on DAVE Network. In this preferences pane click on the "Policies" tab. Under "LAN Manager Authentication" choose the same option mentioned in the ADmitMac instructions above.

NOTE: Making this change will prevent the client from logging into the domain using Kerberos or NTLMv2