PHP Warning

[johannesmutter]

This error message appears every time an appointment is made/ edited …

PHP message: PHP Warning: call_user_func_array() expects parameter 1 to be a valid callback, class 'WC_Appointments_Integration_GCal' does not have a method 'sync_calendar' in /var/www/html/wp-includes/plugin.php on line 524 PHP message: PHP Warning: call_user_func_array() expects parameter 1 to be a valid callback, class 'WC_Appointments_Integration_GCal' does not have a method 'sync_appointment' in /var/www/html/wp-includes/plugin.php on line 524" while reading response header from upstream, client: 2a00:23c4:3700:2000:4d91:60d8:7af:e10, server:

[Željan Topić]

You probably did an override of 'WC_Appointments_Integration_GCal' class, because those two methods are not present in core plugin files anywhere.