Unknown sticky Problem after MySQL upgrade
PROBLEM
I think your problem stems from a change in sql_mode.

sql_mode is blank in MySQL 5.5
It is not blank in MySQL 5.6

SOLUTION
Run this command
SET GLOBAL sql_mode = '';

Then, add this to my.cnf to keep sql_mode blank should you restart mysqld
[mysqld]
sql_mode = ''

You do not need to restart mysqld since you set it globally
GIVE IT A TRY !!!