- At Merchant Center please note the line number in the error message.
- Open your XML feed file and check the aforementioned line.
- Note the tag used in this line.
- At your Magento backend open feed profile you used to create this feed file.
- Find the tag you noted, click it then 'Add Modifier' > 'HTML escape'
- Save the feed profile, generate and upload as needed.
Exact sequence to add 'HTML escape', using 'description' attribute as example: