This error message is normally to do with passwords.
** I use Novell so it may be different if you are backing up Windows NT **
I have had this one before.
You have probably not entered in a correct username and password when you are trying to backup either the server or the NDS. In the versions of Arcserve I have, I enter this information when selecting the Source.
Remember also that you have to enter a valid username and password if you want a backup results printout.
The other possibility is that:
1) You are using your own username and password when setting up your backup.
2) You changed your password.
3) You did not go into Arcserve and change the password there.
If you are doing this, create a seperate user for doing the backups.
Let me know if this is any use.