ERROR: The controller for path '/-sll/error_page.gif' was not found or does not implement IController.

MEANING: This is an artifact of the true error which is typically a 500 - Internal Server Error, or shows as a 404 error in Cloud Retailer and is a performance problem with the Magento server.  Be sure the server meets the minimum requirements.  See here: Magento Server Minimum Requirements

ERROR: Db validation exception when trying to add a category from Magento to Cloud Retailer: Code : The field Code must be a string or array type with a maximum length of '20'.

MEANING:  There's a category in Magento with a name that's longer than 20 characters.  Cloud Retailer has a limit on category names and takes categories from Magento and moves them into CR.  Trim your Magento category descriptions.

ERROR: The following attributes sets: <Attribute set name> are in active use in CloudRetailer for products but do not have a corresponding attribute set id in Magento.

MEANING: Prior to synchronizing data from CR to Magento you need to manually create attribute sets in Magento that match the attribute sets in Cloud Retailer.

ERROR: Error for <Product name>: System.AggregateException: Once or more errors occurred. --> CloudRetailer.Integrability.Magento.SwaggerException`1[CloudRetailer.Plugins.Integrability.Magento.ErrorResponse]: 400 error status {"message":"URL key for specified store already exists."}

Likely MEANING: Products cannot have duplicate descriptions in Magento but they can in Cloud Retailer.  If you have a product description that's the same across two products you'll need to change one of them in CR in order for the synchronization to process correctly.  This error can have other meanings as well.

ERROR: No technical error, however, product quantity on hand data is not synchronizing with the site.

Likely MEANING: Check the price of the product for the location that is being synchronized.  If the price is less than or equal to zero then we skip sending data on that product to Magento as it does not allow for this.