I also encountered this issue, when our store upgraded to magento CE 1.4.2. Whenever I edit and save the customer and this ajax request is made
“ https://example.com/index.php/admin/customer/validate/id/1/?isAjax=true “
And I get the following response from it :
“ Recoverable Error: Argument 1 passed to Mage_Customer_Model_Attribute_Data::factory() must be an instance of Mage_Customer_Model_Attribute, instance of Mage_Eav_Model_Entity_Attribute given”
When I disable Amasty_Customerattr ( Customer Attributes ) module, problem gets fixed.
Solution mentioned above works too, but I also agree with you that this is not the good solution.
Edit : Disabling developer mode in index.php fixed the problem.