I don't know where that error comes from but cleaning the fields after the order is a good idea.
That can be done quite easily by adding the code:
$app->setUserState( HIKASHOP_COMPONENT.'.checkout_fields_ok',0);
$app->setUserState( HIKASHOP_COMPONENT.'.checkout_fields',null);
after the code:
$app->setUserState( HIKASHOP_COMPONENT.'.coupon_code','' );
$app->setUserState( HIKASHOP_COMPONENT.'.cc_number','');
$app->setUserState( HIKASHOP_COMPONENT.'.cc_month','');
$app->setUserState( HIKASHOP_COMPONENT.'.cc_year','');
$app->setUserState( HIKASHOP_COMPONENT.'.cc_CCV','');
$app->setUserState( HIKASHOP_COMPONENT.'.cc_owner','');
$app->setUserState( HIKASHOP_COMPONENT.'.cc_valid',0);
$app->setUserState( HIKASHOP_COMPONENT.'.checkout_terms', 0 );
in the file components/com_hikashop/controllers/checkout.php
That should hopefully fix your problem.