Strange issue regarding my.cnf
You need to change the my.cnf file to include the single line under [mysqld] in your file

max_allowed_packet=16M


now restart the MySQL service and you are done.

You can see its current value in mysql like this:

SHOW VARIABLES LIKE 'max_allowed_packet'


You can try changing it like this, but it's unlikely this will work on shared hosting:

SET GLOBAL max_allowed_packet=16777216;


Also please read this: http://dev.mysql.com/doc/refman/5.1/en/packet-too-large.html