No errors in the server logs. All JavaScript loading OK.
Did notice that the WP_Piwik plugin was giving an incorrect SiteID though.
Corrected that but still couldn't register visitors.
In the end I deleted the entire database, all the piwik files, the most recent version of WP_Piwik plugin.
Reinstalled the old 1.1.2 version of Piwik and the old version of the WP_Piwik plugin that I still had sitting on my hard drive.
Now it works! (but I can't upgrade any of them).
If I update any of the installed files, it again stops working.
Oh well, at least it's working again!
Did notice that the WP_Piwik plugin was giving an incorrect SiteID though.
Corrected that but still couldn't register visitors.
In the end I deleted the entire database, all the piwik files, the most recent version of WP_Piwik plugin.
Reinstalled the old 1.1.2 version of Piwik and the old version of the WP_Piwik plugin that I still had sitting on my hard drive.
Now it works! (but I can't upgrade any of them).
If I update any of the installed files, it again stops working.
Oh well, at least it's working again!