I found out that it is due to the ServerMonitor plugin. Maybe you can create a bug report and thank developers, and also ask them to not rely on this function, if it is possible, as it would make their plugin a bit easier to use Issues · Invision-Technology-Soultions/ServerMonitor · GitHub
Since the update of PHP 5.3 on 05.05.33
I Have also the error: "PHP function rrd_fetch does not exist Please Ensure you have installed PHP RRD functions.: