Using Softaculous to install Piwik, there was a failure due to a stated conflict in writing install.php. I chose the option to overwrite. That caused all attempts to open my site to go to a Piwik page.
Changing the install.php to be the same wording as another site I have, gives this:
Fatal error: Class ‘\MPL\Publisher\DownloadWidget’ not found in /home/xxxxxx/public_html/wp-includes/class-wp-widget-factory.php on line 43
Now what?
I think you’d better try to install Piwik standalone.
Then ask them for support on installation if things outside Piwik are wrong.
