Snow leopard server will not let me log in

I have a mac mini running OS X Server Snow Leopard [I know, ancient system]. The server is used in a home setting and is set up as a local server with three client users plus an administrator user account. At least one of the client users (my account) also has administrator rights. I know the user names and passwords for all these accounts, but when trying to log in to the server directly, the server will not accept any of these users' log-in credentials.

Logging into the server and accessing shared folders is no problem when logging in as a client from another Mac.

When trying to log in to the server - either via keyboard and screen physically connected to the server, or when using screen sharing - the following happens: It thinks for a very, very long time, then shakes and denies access. While it's shaking sometimes I can see the User Name and Icon briefly - confirming the user exists. However, if I intentionally use an incorrect user account or password - it shakes and denies right away.

I have no clue how to log into the Server, so I can access the whole Server and not just the shared folders, when it is behaving this way. Anybody here have any ideas of what I should do?