Zend certified PHP/Magento developer

403 forbidden error while updating a product

While updating a product I am getting the below error

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So when I checked the ajax request it is returning a 403 forbidden error. The error is happening for only some products. Other products are getting updated. The ajax request is admin/catalog/product/validate/id/3209/type/simple/store/0/set/4/key/1bf9ffe87ef4731b231675a326e8040fefeac64590034673g3b4527y3fba8c79/?isAjax=true