These errors are related to Private Messaging add-on which is set in flat-files mode.
For inexperienced users we always recommend to set this add-on in mySQL mode only. This can be done modifying addon_pmail_options.php - setting
$pmMysql=TRUE;
And there should be a file in the forums folder called addon_pmail_funcdb.php.
In other case, I don't understand how you are trying to "add a user" because it's just a
regular registration process... you don't need to rack phpMyAdmin.