The "Input data tag is invalid or missing" error usually reffers to a missing value. Usually the next line in that error message will tell you exactly what is missing.
Below is a list of elements that could be missing and what they mean.
a. Item.PrimaryCategory.CategoryID: This means that the local category of the product you tried to sinchronize is not mapped to any ebay category. This error should only be received when trying to manually sinchronize a product from the "Ebay" tab on the products back office page.
1. Go to the the "Associations" tab on the products page.
2. Look at the "Default category" and see what category is selected in the drop down menu.
3. Go to the Ebay module page and map that that category to a ebay category and set it's item specifics(click here for information on how to do that).
4. Resinchronize the product from the product's page on the ebay tab.