The importer is designed to be run on an empty database; by "restarting" it you're starting the process from the beginning, which the data you're importing exists...
When you "restart" you need to purge the database and run createtables.php again to start with a fresh database.
It sounds like the "page not found" error is your browser timing out, this was common in beta testing with IE as its timeout value is much shorter than in FireFox.
