After hours of searching the manual and the internet, I found the solution thanks to the german Book "Prozessmodellierung mit ARIS®", found on SpringerLink on page 42-43.
If you created a project by yourself, you'll be signed it as user "system" and there is a filter selected. In my case it was the filter "demo database".
To change this, go to View => Options => Login => Filter (click on the button next to the filters name) and select "General" (or similar)
According to the german menu: Ansicht => Optionen => Anmelden => Filter and select "Gesamtmethode"