Actually found out that another plugins/SiteManager/Controller.php file was also empty – some glitch from Filezilla, I don’t know. Sorry about this, nothing to do with Piwik.
So now I have the Piwik dashboard running and all, only problem, overnight no visits are reported so far. I know for a fact that there has been visits to my site. If I look at piwik_log_visit, the table is empty. The only log table for which there are data is piwik_log_action. All piwik_logger_* tables are empty, so no hint as to what is not working.
(I’m liking this thing so far, I really wish to make it work, nice to know we don’t have to rely on a commercial party to manage our own stats.)
Edit: I should have mentioned too that I don’t have access to server settings, in case that matters.
Edit2: I activated DBstats plugin – and when I go to the tab “Database usage”, I get “Mysqli prepare error: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ‘?’ at line 1”. Sigh.
Edit3: Ok I replaced MYSQLI with PDO_MYSQL as suggested above, and now the Database usage tab display properly. I can confirm that only piwik_log_action and piwik_log_link_visit_action contains data. Is this normal behavior? When should I expect the Dashboard to display anything at all (currently it displays all empty despite Piwik having been running overnight.)