I narrowed it down. The problem seems to be the epiphany-browser. I switched to chromium and now all modules are being loaded.

My Setup:
Raspberry Pi 2
Raspbian: 9.4
node: 10.9.0
npm: 6.2.0
MagicMirror: 2.4.1

Since I didn’t get electron to work, I’m using MM in the serveronly mode. After my RPi boots and the graphical env is loaded, the following commands are executed from ~/.config/lxsession/LXDE/autostart:

sh /home/pi/mm.sh

The mm.sh file contains:

exec > /var/log/mm.log
exec 2>&1
cd /opt/MagicMirror
node serveronly &
sleep 180
epiphany --profile /home/pi/.config -a http://localhost:8080 &
sleep 60
DISPLAY=:0 xte 'key F11'

When i access MM from my laptop everything is fine, but from the RPi the calendar module seems not to be loaded.
In the logfile /var/log/mm.log I see the following messages:

** Message: console message: [native code] @0: Starting module: alert

** Message: console message: [native code] @0: Start updatenotification

** Message: console message: [native code] @0: Starting module: clock

** Message: console message: [native code] @0: Starting module: currentweather

** Message: console message: [native code] @0: Starting module: weatherforecast

** Message: console message: [native code] @0: Starting module: newsfeed

** Message: console message: [native code] @0: All modules started!

Does anyone has a clue why the calendar module is not loading?

EDIT: I changed the script so that it contains all the commands.