I've made the upgrade described as summary, and here is the result :
I'll try to figure out what went wrong, but if someone have any clues...
Keywords: update, upgrade
Website URL? Do you have the fix from ?
Try uninstalling Flash completely. Run a registry cleaner. And then reinstall the latest version of Flash.
Also, clear/delete/disable the browser cache and restart the browser.
As a last resort, TRUNCATE TABLE piwik_user_dashboard;
Closing, as forums are more appropriate for further discussion.