The reason is MariaDB database backend which comes installed with Kali Linux:

MariaDB is a community-developed fork of the MySQL relational database management system intended to remain free under the GNU GPL.

This database backend doesn't allow root@localhost user access from applications.

I fixed the issue by creating a new database user, bwappu, with granted priviledges for the bwapp database:

GRANT ALL PRIVILEGES on bwapp.* to 'bwappu'@'localhost';

The reason is MariaDB database backend which comes installed with Kali Linux:

MariaDB is a community-developed fork of the MySQL relational database management system intended to remain free under the GNU GPL.

This database backend doesn't allow root@localhost user access from applications.

I fixed the issue by creating a new database user, bwappu, with granted priviledges for the bwapp database:

GRANT ALL PRIVILEGES on bwapp.* to 'bwappu'@'localhost';

Also, see this post: https://stackoverflow.com/questions/28068155/access-denied-for-user-rootlocalhost-using-password-yes-after-new-instal

The reason is MariaDB database backend, which comes installed with Kali Linux:

MariaDB is a community-developed fork of the MySQL relational database management system intended to remain free under the GNU GPL.

This database backend doesn't allow root@localhost user access with empty ("") password from applications.

I fixed the issue by creating a new database user with granted priviledges for the bWAPP database. Everything works just fine then.

The reason is MariaDB database backend which comes installed with Kali Linux:

MariaDB is a community-developed fork of the MySQL relational database management system intended to remain free under the GNU GPL.

This database backend doesn't allow root@localhost user access from applications.

I fixed the issue by creating a new database user, bwappu, with granted priviledges for the bwapp database:

GRANT ALL PRIVILEGES on bwapp.* to 'bwappu'@'localhost';