Simple solution for ‘The value specified in the URL Key field would generate a URL that already exists.’?

Magento 2.4.2-p1
Smartwave Porto theme
extensions from Amasty

We have done a migration using the Magento Migration Tool and now we constantly get Error when updating categories or products such as right now when added new products to a test category (which only had 1 test product in there when doing this):

The value specified in the URL Key field would generate a URL that already exists.

To resolve this conflict, you can either change the value of the URL Key field (located in the Search Engine Optimization section) to a unique value, or change the Request Path fields in all locations listed below:

  • test-new-2018.html

    Argument 1 passed to MagentoCatalogModelCategoryFileInfo::removeStorePath() must be of the type string, array given, called in /home/OURWEBSITE/public_html/vendor/magento/module-catalog/Model/Category/FileInfo.php on line 167

I would normally go and change the URLs of the products mentioned in the Error message but we can’t be doing that all the time. Why does this happen and how to best resolve this once and for all?