I have multiple machines with Lion installed (Mid 2010 MacBook Pro, 2.66 GHz Mac Mini & a late 2010 iMac) and several more runing Snow Leopard. I cannot make AFP connections to any Lion machine from any other machine, Lion or not. At the same time I can make Screen Sharing connections and several other types of connection to the Lion machines, and can make any type of connection (including AFP) from the Lion machines to the Snow Leopard machines.
After I enter my password, attempted AFP connections to Lion invariably draw the "This file server will not allow any additional users to log on" alert.
Yet the target Lion machine shows "Number of users connected 0". I don't know how to find the crash log, but no AFPFileServer process appears in the Activity Monitor's All Processes list on the 2 Lion machines I checked, so this might be the problem. I've tried repairing the file system & permissions, to no avail.
=> Spettinger's workaround - switching AFP off and SMB on - does indeed allow me to make file sharing connections to my Lion machines. Thanks spettinger!
But, how can this be? There is nothing unusual about my configuration: its all recent Apple hardware & system software, with no 3rd-party protocols & no hacking. Why doesn't everyone who shares files have this exact problem?