Do I have to empty the database of a fresh civicrm install, before I import a dump from another server? (and a question about mysql 5.7 dump ->8.0)
Are you using phpmyadmin to export the old one? You can check the box for ADD DROP TABLE which should solve this. There should also be an option to disable key checks but the drop table might add that automatically.
Using mysqldump on the command line it does this by default, and also by default it disables key checks.