Below are explanations as to why this error is happening.
Property 'weight' for product with sku of is missing - invalid msg! API method 'product_stock.update':Product not exists. Failed to update stock for
A) Magento is unable to respond with the exact SKU because it's not in the Magento db. Most likely, the product is failing because it's not meeting criteria to publish a new product in Magento. Inventory Only Action, will have this error if you're trying to update items that are not in Magento.
B) Error: PM operational.logger.SystemLogger writeLog
ALL: exception.CommunicationException: com.sun.xml.ws.client.ClientTransportException: HTTP transport error: java.net.SocketException: Connection reset
Resolution: Magento 1.x, requires TLS 1.0 in order to work.
Ticket Reference: http://supportcenter.kosmoscentral.com/helpdesk/tickets/5911