In some cases,

The categories entity had an empty request_path.

Go to the database and with the help of this command find the categories or products entities that have an empty request_path

SELECT * FROM `url_rewrite` WHERE request_path='';

Delete all those entries and use the cache clean and flush command:

php bin/magento cache:clean
php bin/magento cache:flush

Revisit the url, and the homepage will work fine once again.