Searched the forum if this was reported but didn’t find anything.
Original warning message:
WARNING: /piwik/core/Plugin/ReportsProvider.php(166): Warning - usort(): Array was modified by the user comparison function - Piwik 3.2.0 - Please report this message in the Piwik forums: http://forum.piwik.org (please do a search first as it might have been reported already)
A quick google search seems to point out that this could be a PHP bug caused by the way the function is written.
Could you open a new Github issue and also mention your PHP version and operation system.
I have also read that PHP 7 fixes this issue
we have the same problem. Update from Piwik 3.0.4 to 3.2.0 was done before the update from PHP 5.6 to 7.0.
There was no information beforehand that PHP 7+ is needed. But there was an exception when updating.
That could be the reason for this Warning/Exception.
Now PHP 7.0 is installed but the Warning is still there.
Have you tried updating all plugins since you updated piwik? Maybe an older version of an third party plugin isn’t compatible with the latest piwik version.
You could also try out temporarily disabling third party plugins to see it it fixes the issue.
We had this problem and it seems that upgrading PHP to latest version has fixed it. Latest of 7.0 or 7.1