In certain cases, AD (Active Directory) authentication to AFP shares (Apple File Protocol; Apple file sharing) stops working on Mac OS X Lion Server.
If this occurs, first make sure that the server is still bound to AD, and that you can log in directly on the server with an AD account.
Assuming the AD binding is fine, next, open the Server Admin Tools for Lion — this is a separate download you can find here on Apple’s site.
Connect to your server using Server Admin, click the name of the server in the left pane, and then click the Access tab. Make sure that AFP is set to “Allow all users and groups.”