You are attempting to execute a procedure from an EXECUTE statement, but it has been redefined since the PREPARE statement was run. You must use the PREPARE statement again on the EXECUTE PROCEDURE statement.
For example, the following sequence of code would cause this error:
$prepare pr_stat from 'execute procedure testproc()'; ... /* drop procedure testproc create procedure testproc() ...... [same application or different] */ ... $execute pr_stat;/* triggers error -721 */