this works http://support.apple.com/kb/HT4700 ... at first..
it worked for my lion mid2010 mpb when i first had the same problem, recently upgraded to mountain lion with no issues with afp connection.
but latest mba i got 2 weeks ago, the workaround worked for as long as i stay connected. after disconnection, repetition of the workaround doesn't seem to work anymore.
try it to see if you get the same problem?