After upgrading our machines at home to Lion, I discovered that AFP file sharing no longer functioned. I would get various errors when trying to connect from one machine to another. The errors were things like "too many users connected." Well, after much research (apparently there are many problems with Lion's implementation of AFP and SMB), I found that deleting the preference file referenced below and rebooting fixed my file sharing issues.
I had what seems to be the same problem, and I think I solved it: In /Library/Preferences/com.apple.AppleFileServer.plist there were two keys, maxConnections and maxGuests, that were set to 0. Looked like this:
I changed the values to 10, and it seems to be working now.