Correct content of zotero.desktop as follows:
[Desktop Entry]
Type=Application
Encoding=UTF-8
Name=Zotero
Comment=Bibliografisch programma
Exec=/home/lieven/ICT/Configuratie\ en\ Setup/Zotero_linux-x86_64/zotero
Icon=/home/lieven/ICT/Configuratie\ en\ Setup/Zotero_linux-x86_64/chrome/icons/default/default48.png
Categories=Utility
Terminal=false
Path with space should contain a backslash.
Also make zotero.desktop executable through Terminal:
sudo chmod +x /usr/share/applications/zotero.desktop
PS: No need to get into system files, *.desktop with same success can be placed in ~/.local/share/applications. In this case, command to change permissions of an executable file will look like this:
chmod +x ~/.local/share/applications/zotero.desktop