Adding group rights in db2 for a stored procedure
I ran into a similar problem and while this wasn't what happened here, this answer may help others running into this. 

Make sure the OS group mygroup is all lowercase. DB2 on AIX does not recognize uppercase letters in the group name and will fail to validate a user belongs to a group if that group name has uppercase letters. eg:

# id useraccounthere
uid=xx(useraccounthere) gid=yyy(mygroup) &lt;- this works

# id useraccounthere
uid=xx(useraccounthere) gid=yyy(MyGroup) &lt;- this does not work


This is documented here: DB2 UDB Security