Not sure if you missed any code but <% control Counter %> doesn't seem to point to any functions. However it looks like pagecount() outputs the counter so you would use $pagecount in the template.

Since an new serverupdate i couldnt build the db with the pagecounter. Databases were not builded. The PageCounter.php code was the problem <? should be <?php

good working code:

PageCounter.php
____________________________

<?php
class PageCounter extends DataObject {
static $db = array(
'Counter' => 'Int'
);
static $has_one = array(
'Page' => 'Page'
);
}
?>

____________________________
added in page.php (code was OK)
replace from: class Page_Controller
till: parent::init();
____________________________

class Page_Controller extends ContentController {

public static $allowed_actions = array (
   );
   
public function init() {
$pagecounter = DataObject::get_one("PageCounter","PageID='$this->ID'");
if(!$pagecounter){
$pagecounter = new PageCounter();
$pagecounter->PageID=$this->ID;
}
$pagecounter->Counter = $pagecounter->Counter+1;
$pagecounter->write();

parent::init();
      
// Note: you should use SS template require tags inside your templates
// instead of putting Requirements calls here. However these are
// included so that our older themes still work
Requirements::themedCSS("layout");
Requirements::themedCSS("typography");
Requirements::themedCSS("form");
}

_______________________________
second added in page.php right before the end:
}
?>
_______________________________

public function pagecount(){
$pagecounter = DataObject::get_one("PageCounter","PageID='$this->ID'");
return $pagecounter->Counter;
}

________________________________
Then my Themes/ Page.ss added just where i wanted to show
________________________________

$pagecount

________________________________

All works great now.

Based on your solution, is there a way to list the 5 most popular pages with page title and link?

For displaying the most popular posts check out this article