If the Windows Server is not a domain controller (Windows AD Domain) there is no central repository for user authentication.
And in a workgroup environment in order for client machines / users to access shares on the "SERVER" they will have to have a username and password on the "SERVER" that is the same as the one they are using to log on locally to their machines in order for passthrough authentication to work. Otherwise they will be prompted for credentials at which point they can check the box to remember the credentials and they will be preserved in the credentials vault. If anything has changed regarding those credentials you may need to delete and re-establish them. Also double check and confirm that the WORKGROUP name is the...