Your PHP is configured to stop running a script after some time, generally 30 seconds. When Matomo (Piwik) is used on a websites with few thousands visits per day, then this limit might not be high enough. To fix this, modify in your php.ini the max_execution_time value to zero (which means no time limit):

max_execution_time = 0

This will ensure that Matomo always has enough time to process your analytics reports.

To find where is your php.ini file on your server, you can follow the following steps: create a test.php file and add the following code:

 <?php phpinfo(); ?>

and open it in browser, it will show the file which is actually being read by PHP running on your webserver. It will also show your currently set max_execution_time value.