Some hosting servers have the getmypid function disabled for security reasons. We have some checks to see if this function is enabled or not but they aren't being used before it is called in this scenario resulting in an error:

[29-Mar-2023 10:30:58 UTC] PHP Fatal error: Uncaught Error: Call to undefined function Piwik\CliMulti\getmypid() in /home/wp/disk/wordpress/wp-content/plugins/matomo/app/core/CliMulti/Process.php:125
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[29-Mar-2023 10:30:53 UTC] Matomo: Scheduled tasks archive data

Reported by a couple of customers here:
matomo-org/matomo-for-wordpress#660