v2.1.x Fehler im Warenkorb bei Klick auf Versandlink

Hallo,

wenn ich im Warenkorb auf Versand klicke unter der Summe.

Erhalte ich folgende Fehlermeldung im PopUp:
USER ERROR(256): "check_data_type validation failed. Dump: , integer expected, but string detected"

Code:

#3    File: /domain/htdocs/gm/inc/check_data_type.inc.php:27 (Details)
    Function: trigger_error('check_data_type validation failed. Dump: , integer expected, but string detected', 256) (Details)
    Code:
│    line 25:             else
│    line 26:             {
├─    line 27:                 trigger_error('check_data_type validation failed. Dump: ' . print_r($p_data, true) . ', integer expected, but ' . gettype($p_data) . ' detected', $p_error_level);
│    line 28:                 return false;
│    line 29:             }
#2    File: /domain/htdocs/system/core/BaseClass.inc.php:146 (Details)
    Function: check_data_type('', 'int', false, 256, '') (Details)
    Code:
│    line 144:         }
│    line 145: 
├─    line 146:         $t_valid = check_data_type($p_variable_value, $t_type, $t_strict, $t_error_level, $t_class_name);
│    line 147:         
│    line 148:         return $t_valid;
#1    File: /domain/htdocs/system/core/BaseClass.inc.php:56 (Details)
    Class: BaseClass (Details)
    Method: check_data_type('content_group_id', '') (Details)
    Code:
│    line 54:         if(array_key_exists($p_variable_name, $this->validation_rules_array))
│    line 55:         {
├─    line 56:             if($this->check_data_type($p_variable_name, $p_variable_value) == false)
│    line 57:             {
│    line 58:                 return false;
#0    File: /domain/htdocs/popup_content.php:27 (Details)
    Class: BaseClass (Details)
    Method: set_('content_group_id', '') (Details)
    Code:
│    line 25: if(isset($_GET['coID']))
│    line 26: {
├─    line 27:     $coo_popup_content_view->set_('content_group_id', $_GET['coID']);
│    line 28: }
│    line 29: if(isset($_GET['lightbox_mode']))
Request: GET /popup_content.php?coID=&lightbox_mode=1 (Details)
Session:  (Details)
    [..]
    - payment: ### Session entry not present ###
    - shipping: ### Session entry not present ###
    - cartID: ### Session entry not present ###
    - sendto: 1
    - billto: ### Session entry not present ###
Get:  (Details)
    - coID: 
    - lightbox_mode: 1

Wie kann ich das Problem lösen?

Erweiterungen wurden eigentlich keine installiert.

Danke & Gruß