If you change the domain for your piwik server, the plugin has the potential to freeze up your login and there is no way to recover from the GUI.
1) Configure wp-piwik to use e.g. stats.somedomain.com (show stats on dashboard)
2) Change piwik server DNS e.g. stats.anotherdomain.com
3) Sign in to your WordPress site (site won't load)
1) go to a page on your wordpress and click edit (now your in the site). DO NOT GO TO DASHBOARD
2) Go to plugins and delete wp-piwik
3) go to phpMyAdmin, Workbench, or your query tool of choice and delete piwik options by running this script:
-- start script
DELETE FROM wp_options WHERE
wp_options LIKE '%wp-piwik%'
-- end script
it will delete two records.
4) install wp-piwik and reconfigure.
1) When plugin is deleted, delete configuration in wp_options
2) Have a link (or button) to delete configuration data
3) Have a link (or button) to update piwik server
If I go into the DB through MySQL Workbench or phpMyAdmin and try to update the settings manually I get the following error
"Fatal error: Access to undeclared static property: wp_piwik::$bolWPMU in /home/.../wp-content/plugins/wp-piwik/update/80403.php on line 15"
I discovered this by accident as I just mess around on WordPress to see what all the fuss is about. I might run important sites on it soon, not sure yet. I would probably have been more careful in migrating the DNS records on a more important site. But the bug is found and I hope my notes help you out in improving this great plugin.