we have seen recently that the "Online customers" report is always empty + also the column customer_id in table report_viewed_product_index is always NULL
I know because on our DEV server the column customer_id is always a number (never NULL) ....
What setting in Magento can influence this behavior? Has anyone seen this before?
I don't know what your problem might be, but I can explain how the values end up in that table.
when a product page is viewed, the event catalog_controller_product_view is dispatched.
This event is observed by Mage_Reports_Model_Event_Observer::catalogProductView.
This method just instantiates a model and saves record in the table report_viewed_product_index
Mage::getModel('reports/product_index_viewed')
->setProductId($productId)
->save()
->calculate();
In the _beforeSave method for the model Mage_Reports_Model_Product_Index_Viewed the visitor id and customer id are set.
Check Mage_Reports_Model_Product_Index_Abstract::_beforeSave.
I wasn't able to reproduce your issue, but I thought that the stacktrace may help you debug the issue.
