Shamim's Plugins

You can add a default value for subject and hide using css.
Add following code in your theme’s (child theme’s if you are using) functions.php

add_filter( 'fep_form_fields', function( $fields ){
	if( isset( $fields['message_title'] ) ){
		$fields['message_title']['value'] = 'Default Value for all message subject';
	}
	return $fields;
});

Then add .fep-form-field-message_title{display:none} in Front End PM PRO > Settings > Appearance > Custom CSS

Still getting same error.

Sorry for the late reply for weekend.
I tried with admin account and working.
But i could not login using subscriber account, so i could not test that. Can you please check that you can login with subscriber account you provided me? It is showing by cookies is blocked which is not. I tried with multiple browser (Safari, chrome, firefox) and showing same error message.

Yes, pass value when creating form field with that meta value.

Please add following code in your theme’s (child theme’s if you are using) functions.php

add_filter('fep_directory_table_columns', function( $columns ){
    unset($columns['name']);
    return $columns;
});

instead of $user = wp_get_current_user(); use $user = get_userdata( $userid );

Are you logged in as same user as that admin? You cannot send message to yourself.
Log in as another user and try again.

Let me know.