Apache2 crash on boot, but not when started manually

I got a wired problem that I don't understand at all:

I have a home server with an Apache2 installation on Ubuntu 18.04. My problem is that on system reboot, Apache2 keeps crashing.

systemctl status apache2 gives me:

● apache2.service - The Apache HTTP Server
Loaded: loaded (/lib/systemd/system/apache2.service; enabled; vendor preset: enabled)
Drop-In: /lib/systemd/system/apache2.service.d
         └─apache2-systemd.conf
Active: failed (Result: exit-code) since Sat 2020-05-23 13:42:31 CEST; 3min 52s ago
Process: 1183 ExecStart=/usr/sbin/apachectl start (code=exited, status=0/SUCCESS)
Main PID: 1224 (code=exited, status=1/FAILURE)

mai 23 13:42:30 potato-server systemd[1]: Starting The Apache HTTP Server...
mai 23 13:42:30 potato-server systemd[1]: Started The Apache HTTP Server.
mai 23 13:42:31 potato-server systemd[1]: apache2.service: Main process exited, code=exited, status=1/FAILURE
mai 23 13:42:31 potato-server systemd[1]: apache2.service: Failed with result 'exit-code'.

What's super wired is that when I do a manual sudo systemctl start apache2, the server starts. And apache2ctl configtest gives me:

Syntax OK

Any idea of where the problem can come from ?