I try to install Piwik on a system with Debian and Nginx.
Whenever I send the form with the database credentials in the setup process the same database setup page is loaded again with the default values. I had Piwik configured to write a log file and there I can read
SQLSTATE  Access denied for user 'www-data'@'localhost' (using password: NO) but www-data is not the username I entered in the form.
I tried to enter the complete IP address from my server but in the log files, it is again localhost.
Why is this happening and what can I do?