This product belongs to a different category, so the category has been changed

The warning may be returned when eBay resets the category to the one that is linked to the catalog.

It happens when the product details that you include in your listing do not comply with the Primary category that you set. So, eBay reset the category and returned the warning. 

To get over the warning, you may try to list the item in the category Other or edit product details to make them match the category you want.  

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