Please check your error log files.

No record since the update

Are the same amount of actions / pageviews recorded?

No. See image. There is just 10 recorded visitors with 100 actions during last 24 hours.
Before 22th October (when I do the update) there was around 300 visitors per 24 hours.
Statistics from Google Analaytics shows still around 300 visitors per day. But Piwik record only few of them (11-35)

Can you try to debug the tracker? I reckon there will be some kind of error during the tracking requests. Here is an article re how to enable logging: http://developer.piwik.org/api-reference/tracking-api#debugging-the-tracker . You will most likely also have to do the second step and enable logging to file or database as described here: http://piwik.org/faq/troubleshooting/faq_115/

I'd recommend to enable tracker logging until you see an error in the logs and then disable it again to prevent from exposing any logging data to your users.

I have done it.
In tmp/logs/piwik.log I see only few visitor from few sites.
However, I can not see there much more visitors, including those visits made by me.

@adminkpr how did you upgraded piwik? Did you downloaded it from here? https://github.com/piwik/piwik/releases

I have upgraded piwik online through the admin on my server.

Hmm. I had such an error one of time also. The counter visitors were returning, which had the piwik.js already cached. Because new visitors couldnt load the piwik.js it didnt worked anymore

If there are only a few visitors in the log and no errors it sounds like the requests are not making it to your server somehow. Can you maybe check your apache or webserver log and PHP error log? If you cannot see your visits there, can you maybe open the developer tools in the browser (in case you are familiar with it) and see if it actually sends a tracking request to your Piwik? Maybe you have an ad/privacy blocker enabled or Do Not Track?

There is no error records in the PHP error logs. I have even downloaded https://github.com/piwik/piwik/releases and upload it again. I really do not why, but at least 4 website has extreme decline of visitors (nearly zero) when compared before and after upgrade. The rest websites has less or more the same amount of visitors.

@tsteur Can you email me at admin(at)kpr.eu. I will give you password to access my piwik and you can look on it.

Email is hello at piwik.org.

BTW: The downloads in releases should not be used, instead please use http://piwik.org/download for stable releases and http://builds.piwik.org/ for betas and RC releases.

@mattab re the GitHub releases: Can we attach the built zip there where tests etc are removed or alternatively disable downloads there?

It's possible to add files to the release: https://help.github.com/articles/creating-releases/

If you'd like to include binary files along with your release, such as compiled programs, drag and drop or select files manually in the binaries box.

I think the releases are created with a script, but I'm sure there's also an API for that. I will create an issue for it

This issue is fixed by #9142 and a duplicate of #9130

@adminkpr It should be fixed by overwriting your piwik.js in the Piwik directory with this: https://raw.githubusercontent.com/piwik/piwik/9130/piwik.js

The website overwrites JSON caused by http://mootools.net/core/docs/1.5.2/Utilities/JSON

@adminkpr thx for providing us access to your instance. You can remove our account again from your Piwik.

BTW: There's another unrelated JavaScript error on your actual website of idSite 10

There is nothing on this link https://raw.githubusercontent.com/piwik/piwik/9130/piwik.js

that was already merged to master. so use https://raw.githubusercontent.com/piwik/piwik/master/piwik.js