9 times out of 10 this error is caused by an insufficiently large max_allowed_packet setting in your MySQL server's my.ini file.
The standard default for MAMP in my experience is 1M, which isn't usually enough for Drupal (especially considering the size of some of the cache strings it generates).
Try increasing that setting gradually to see if the error disappears. I'm sure a quick google search will tell you how to do that in MAMP.