I think that file should not exist and was deleted > 2 years ago. It should be fine to just delete that file.

... seriously? Ok. Heh. BaseEcommerceItem.php and BaseGoal.php too, then, I ass-u-me?

Yep :)

Actually, doing that makes piwik lock after displaying...

"Oops… there was a problem during the request. Maybe the server had a temporary issue, or maybe you requested a report with too much data. Please try again. If this error occurs repeatedly please contact your Piwik administrator for assistance. "

Sorry, forgot to mention you likely need to empty the "tmp/cache" folder in piwik.

I'm pointing finger @tsteur (but in a friendly way) by mentioning this:

#6402 (comment)

IMHO, clearing cache should happen at Piwik's update, upgrade and every time you add or remove a plugin...

I know, in this case the file was removed manually from the file system

Which of course is impossible, since this is running on normal commercial hosting, thus the files are all owned by the webserver process (in this case, nobody/users). This is just how commercial hosting works, for security.

Sometimes there are "tools" where you can enter a path to delete a directory or it lets you browse the files? Is there no way to remove the files from a directory for you? You may want to ping that hoster to add such functionality.

I had this problem as well, and additionally QueuedTracking is not working with error:

PHP Fatal error: Cannot redeclare class Piwik\Plugins\UserCountry\LocationProvider\DefaultProvider in /srv/www/piwik.cidc.com.kh/plugins/UserCountry/LocationProvider/Default.php on line 112

I would say the official Piwik update process is flawed.

Some other big web applications like Piwik first extract the update to a temporary directory, and then running the update "merges" into the production directory. This could delete the old unnecessary files..

In the meanwhile, I think we'll start to unzip the Piwik distribution to a temporary directory and re-install plug-ins. It seems more fail-safe than hunting down all those files that should not be there.

Is a proper update system on the roadmap?

@tsteur, There are more people having the same issue:
https://forum.piwik.org/t/warning-after-update/24748
https://forum.piwik.org/t/undefined-index-idsites-piwik-3-0-2/23571

It seems like all of them are updating from a 2.x version that isn't 2.17. Is it possible there is an bug in the updater?

It is not really a bug. The feature to also update plugins with the core was only added in 2.17. or so. Updating from an earlier version won't update the plugins automatically and may cause trouble.

Maybe it would be helpful in the future if updating to piwik 4 was only possible from the last 3.x version and the updater would only do it in two steps.
I feel like most of the "the-update-broke-piwik"-issues happen to people who update from older 2.x versions.

yeah be good probably. We'd need to see how to implement that when refactoring the updater in #5985 as there are a couple of "problems" with the updater