I had the problem of no tracking data showing up for two sites that usually had lots of visitors. I checked for a solution and found this:
The tracking code for these sites contained a reference to the file piwik.js.gz. During the latest update I was asked to remove a number of files, which I did. (Mainly because I had been asked this a number of times before.) My guess is that this was one of them (no longer have the list of files). We have used Piwik for years and this maybe the reason the tracking code included g.src=u+‘piwik.js.gz’;
So if the Firefox Developer Tools show you this line Loading failed for the <;script> with source “https://www.united.de/piwik/piwik.js.gz”. (look under Console), then this is the reason.
I changed the code to g.src=u+‘piwik.js’;, and everything is working again.
Just my 2 pence…