This are the files in the config folder that your colleague in the Wordpress forum suggested. They were all updated when I updated Matomo to 4.15.2 on October 13th.
And this is where the error happens:
[Tue Oct 24 11:08:08.122639 2023] [authz_core:error] [pid 36952:tid 140125128296192] [client XXX] AH01630: client denied by server configuration: /srv/www/XXX/www/wp-content/plugins/matomo/app/config/global.ini.php
(partly anonymized by replacing with XXX)