Update: Maintenance postponed, I think I found the cause of the error (php function loop).
My website will go down on 30.6.2020 from 10:00 (GMT+2) to 12:00 (GMT+2).
My site is running on a Raspberry PI at home and the Apache2 web server has an issue that makes my site temporarily unavailable untill I reload the server. There is no log that points me to the cause so I am going to migrate the web server from Apache2 to NGINX. Cross your fingers so that everything runs well.